Use templates for issue e-mail subject and body (#8329)

* Add template capability for issue mail subject

* Remove test string

* Fix trim subject length

* Add comment to template and run make fmt

* Add information for the template

* Rename defaultMailSubject() to fallbackMailSubject()

* General rewrite of the mail template code

* Fix .Doer name

* Use text/template for subject instead of html

* Fix subject Re: prefix

* Fix mail tests

* Fix static templates

* [skip ci] Updated translations via Crowdin

* Expose db.SetMaxOpenConns and allow non MySQL dbs to set conn pool params (#8528)

* Expose db.SetMaxOpenConns and allow other dbs to set their connection params
* Add note about port exhaustion

Co-Authored-By: guillep2k <18600385+guillep2k@users.noreply.github.com>

* Prevent .code-view from overriding font on icon fonts (#8614)

* Correct some outdated statements in the contributing guidelines (#8612)

* More information for drone-cli in CONTRIBUTING.md

* Increases the version of drone-cli to 1.2.0
* Adds a note for the Docker Toolbox on Windows

Signed-off-by: LukBukkit <luk.bukkit@gmail.com>

* Fix the url for the blog repository (now on gitea.com)

Signed-off-by: LukBukkit <luk.bukkit@gmail.com>

* Remove TrN due to lack of lang context

* Redo templates to match previous code

* Fix extra character in template

* Unify PR & Issue tempaltes, fix format

* Remove default subject

* Add template tests

* Fix template

* Remove replaced function

* Provide User as models.User for better consistency

* Add docs

* Fix doc inaccuracies, improve examples

* Change mail footer to math AppName

* Add test for mail subject/body template separation

* Add support for code review comments

* Update docs/content/doc/advanced/mail-templates-us.md

Co-Authored-By: 6543 <24977596+6543@users.noreply.github.com>

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+---
+date: "2019-10-23T17:00:00-03:00"
+title: "Mail templates"
+slug: "mail-templates"
+weight: 45
+toc: true
+draft: false
+menu:
+  sidebar:
+    parent: "advanced"
+    name: "Mail templates"
+    weight: 45
+    identifier: "mail-templates"
+---
+
+# Mail templates
+
+To craft the e-mail subject and contents for certain operations, Gitea can be customized by using templates. The templates
+for these functions are located under the [`custom` directory](https://docs.gitea.io/en-us/customizing-gitea/).
+Gitea has an internal template that serves as default in case there's no custom alternative.
+
+Custom templates are loaded when Gitea starts. Changes made to them are not recognized until Gitea is restarted again.
+
+## Mail notifications supporting templates
+
+Currently, the following notification events make use of templates:
+
+| Action name   | Usage                                                                                                        |
+|---------------|--------------------------------------------------------------------------------------------------------------|
+| `new`         | A new issue or pull request was created.                                                                     |
+| `comment`     | A new comment was created in an existing issue or pull request.                                              |
+| `close`       | An issue or pull request was closed.                                                                         |
+| `reopen`      | An issue or pull request was reopened.                                                                       |
+| `review`      | The head comment of a review in a pull request.                                                              |
+| `code`        | A single comment on the code of a pull request.                                                              |
+| `assigned`    | Used was assigned to an issue or pull request.                                                               |
+| `default`     | Any action not included in the above categories, or when the corresponding category template is not present. |
+
+The path for the template of a particular message type is:
+
+```
+custom/templates/mail/{action type}/{action name}.tmpl
+```
+
+Where `{action type}` is one of `issue` or `pull` (for pull requests), and `{action name}` is one of the names listed above.
+
+For example, the specific template for a mail regarding a comment in a pull request is:
+```
+custom/templates/mail/pull/comment.tmpl
+```
+
+However, creating templates for each and every action type/name combination is not required.
+A fallback system is used to choose the appropriate template for an event. The _first existing_
+template on this list is used:
+
+* The specific template for the desired **action type** and **action name**.
+* The template for action type `issue` and the desired **action name**.
+* The template for the desired **action type**, action name `default`.
+* The template for action type `issue`, action name `default`.
+
+The only mandatory template is action type `issue`, action name `default`, which is already embedded in Gitea
+unless it's overridden by the user in the `custom` directory.
+
+## Template syntax
+
+Mail templates are UTF-8 encoded text files that need to follow one of the following formats:
+
+```
+Text and macros for the subject line
+------------
+Text and macros for the mail body
+```
+
+or
+
+```
+Text and macros for the mail body
+```
+
+Specifying a _subject_ section is optional (and therefore also the dash line separator). When used, the separator between
+_subject_ and _mail body_ templates requires at least three dashes; no other characters are allowed in the separator line.
+
+
+_Subject_ and _mail body_ are parsed by [Golang's template engine](https://golang.org/pkg/text/template/) and
+are provided with a _metadata context_ assembled for each notification. The context contains the following elements:
+
+| Name               | Type           | Available     | Usage                                                                                                                                                                                                                                             |
+|--------------------|----------------|---------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `.FallbackSubject` | string         | Always        | A default subject line. See Below.                                                                                                                                                                                                                |
+| `.Subject`         | string         | Only in body  | The _subject_, once resolved.                                                                                                                                                                                                                     |
+| `.Body`            | string         | Always        | The message of the issue, pull request or comment, parsed from Markdown into HTML and sanitized. Do not confuse with the _mail body_                                                                                                              |
+| `.Link`            | string         | Always        | The address of the originating issue, pull request or comment.                                                                                                                                                                                    |
+| `.Issue`           | models.Issue   | Always        | The issue (or pull request) originating the notification. To get data specific to a pull request (e.g. `HasMerged`), `.Issue.PullRequest` can be used, but care should be taken as this field will be `nil` if the issue is *not* a pull request. |
+| `.Comment`         | models.Comment | If applicable | If the notification is from a comment added to an issue or pull request, this will contain the information about the comment.                                                                                                                     |
+| `.IsPull`          | bool           | Always        | `true` if the mail notification is associated with a pull request (i.e. `.Issue.PullRequest` is not `nil`).                                                                                                                                       |
+| `.Repo`            | string         | Always        | Name of the repository, including owner name (e.g. `mike/stuff`)                                                                                                                                                                                  |
+| `.User`            | models.User    | Always        | Owner of the repository from which the event originated. To get the user name (e.g. `mike`),`.User.Name` can be used.                                                                                                                             |
+| `.Doer`            | models.User    | Always        | User that executed the action triggering the notification event. To get the user name (e.g. `rhonda`), `.Doer.Name` can be used.                                                                                                                  |
+| `.IsMention`       | bool           | Always        | `true` if this notification was only generated because the user was mentioned in the comment, while not being subscribed to the source. It will be `false` if the recipient was subscribed to the issue or repository.                            |
+| `.SubjectPrefix`   | string         | Always        | `Re: ` if the notification is about other than issue or pull request creation; otherwise an empty string.                                                                                                                                         |
+| `.ActionType`      | string         | Always        | `"issue"` or `"pull"`. Will correspond to the actual _action type_ independently of which template was selected.                                                                                                                                  |
+| `.ActionName`      | string         | Always        | It will be one of the action types described above (`new`, `comment`, etc.), and will correspond to the actual _action name_ independently of which template was selected.                                                                        |
+
+All names are case sensitive.
+
+### The _subject_ part of the template
+
+The template engine used for the mail _subject_ is golang's [`text/template`](https://golang.org/pkg/text/template/).
+Please refer to the linked documentation for details about its syntax.
+
+The _subject_ is built using the following steps:
+
+* A template is selected according to the type of notification and to what templates are present.
+* The template is parsed and resolved (e.g. `{{.Issue.Index}}` is converted to the number of the issue
+  or pull request).
+* All space-like characters (e.g. `TAB`, `LF`, etc.) are converted to normal spaces.
+* All leading, trailing and redundant spaces are removed.
+* The string is truncated to its first 256 runes (characters).
+
+If the end result is an empty string, **or** no subject template was available (i.e. the selected template
+did not include a subject part), Gitea's **internal default** will be used.
+
+The internal default (fallback) subject is the equivalent of:
+
+```
+{{.SubjectPrefix}}[{{.Repo}}] {{.Issue.Title}} (#.Issue.Index)
+```
+
+For example: `Re: [mike/stuff] New color palette (#38)`
+
+Gitea's default subject can also be found in the template _metadata_ as `.FallbackSubject` from any of
+the two templates, even if a valid subject template is present.
+
+### The _mail body_ part of the template
+
+The template engine used for the _mail body_ is golang's [`html/template`](https://golang.org/pkg/html/template/).
+Please refer to the linked documentation for details about its syntax.
+
+The _mail body_ is parsed after the mail subject, so there is an additional _metadata_ field which is
+the actual rendered subject, after all considerations.
+
+The expected result is HTML (including structural elements like`<html>`, `<body>`, etc.). Styling
+through `<style>` blocks, `class` and `style` attributes is possible. However, `html/template`
+does some [automatic escaping](https://golang.org/pkg/html/template/#hdr-Contexts) that should be considered.
+
+Attachments (such as images or external style sheets) are not supported. However, other templates can
+be referenced too, for example to provide the contents of a `<style>` element in a centralized fashion.
+The external template must be placed under `custom/mail` and referenced relative to that directory.
+For example, `custom/mail/styles/base.tmpl` can be included using `{{template styles/base}}`.
+
+The mail is sent with `Content-Type: multipart/alternative`, so the body is sent in both HTML
+and text formats. The latter is obtained by stripping the HTML markup.
+
+## Troubleshooting
+
+How a mail is rendered is directly dependent on the capabilities of the mail application. Many mail
+clients don't even support HTML, so they show the text version included in the generated mail.
+
+If the template fails to render, it will be noticed only at the moment the mail is sent.
+A default subject is used if the subject template fails, and whatever was rendered successfully
+from the the _mail body_ is used, disregarding the rest.
+
+Please check [Gitea's logs](https://docs.gitea.io/en-us/logging-configuration/) for error messages in case of trouble.
+
+## Example
+
+`custom/templates/mail/issue/default.tmpl`:
+
+```
+[{{.Repo}}] @{{.Doer.Name}}
+{{if eq .ActionName "new"}}
+    created
+{{else if eq .ActionName "comment"}}
+    commented on
+{{else if eq .ActionName "close"}}
+    closed
+{{else if eq .ActionName "reopen"}}
+    reopened
+{{else}}
+    updated
+{{end}}
+{{if eq .ActionType "issue"}}
+    issue
+{{else}}
+    pull request
+{{end}}
+#{{.Issue.Index}}: {{.Issue.Title}}
+------------
+<!DOCTYPE html>
+<html>
+<head>
+    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
+    <title>{{.Subject}}</title>
+</head>
+
+<body>
+    {{if .IsMention}}
+    <p>
+        You are receiving this because @{{.Doer.Name}} mentioned you.
+    </p>
+    {{end}}
+    <p>
+        <p>
+        <a href="{{AppURL}}/{{.Doer.LowerName}}">@{{.Doer.Name}}</a>
+        {{if not (eq .Doer.FullName "")}}
+            ({{.Doer.FullName}})
+        {{end}}
+        {{if eq .ActionName "new"}}
+            created
+        {{else if eq .ActionName "close"}}
+            closed
+        {{else if eq .ActionName "reopen"}}
+            reopened
+        {{else}}
+            updated
+        {{end}}
+        <a href="{{.Link}}">{{.Repo}}#{{.Issue.Index}}</a>.
+        </p>
+        {{if not (eq .Body "")}}
+            <h3>Message content:</h3>
+            <hr>
+            {{.Body | Str2html}}
+        {{end}}
+    </p>
+    <hr>
+    <p>
+        <a href="{{.Link}}">View it on Gitea</a>.
+    </p>
+</body>
+</html>
+```
+
+This template produces something along these lines:
+
+#### Subject
+
+> [mike/stuff] @rhonda commented on pull request #38: New color palette
+
+#### Mail body
+
+> [@rhonda](#) (Rhonda Myers) updated [mike/stuff#38](#).
+>
+> #### Message content:
+>
+> \__________________________________________________________________
+>
+> Mike, I think we should tone down the blues a little.  
+> \__________________________________________________________________
+> 
+> [View it on Gitea](#).
+
+## Advanced
+
+The template system contains several functions that can be used to further process and format
+the messages. Here's a list of some of them:
+
+| Name                 | Parameters  | Available | Usage                                                               |
+|----------------------|-------------|-----------|---------------------------------------------------------------------|
+| `AppUrl`             | -           | Any       | Gitea's URL                                                         |
+| `AppName`            | -           | Any       | Set from `app.ini`, usually "Gitea"                                 |
+| `AppDomain`          | -           | Any       | Gitea's host name                                                   |
+| `EllipsisString`     | string, int | Any       | Truncates a string to the specified length; adds ellipsis as needed |
+| `Str2html`           | string      | Body only | Sanitizes text by removing any HTML tags from it.                   |
+
+These are _functions_, not metadata, so they have to be used:
+
+```
+Like this:         {{Str2html "Escape<my>text"}}
+Or this:           {{"Escape<my>text" | Str2html}}
+Or this:           {{AppUrl}}
+But not like this: {{.AppUrl}}
+```