Add `markdownlint` linter and fix issues. Config is based on the one
from electron's repo with a few rules relaxed.

Thanks to https://github.com/go-gitea/gitea/pull/16788 I can see why our docker container kept restarting when adding the custom mail template example.
[The example template](https://docs.gitea.io/en-us/mail-templates/#example) has an error

```
2021/09/03 10:55:25 cmd/web.go:91:func1() [F] PANIC: template: mail/issue/default:35: function "AppURL" not defined
        /usr/local/go/src/html/template/template.go:374 (0x1563bb8)
        /go/src/code.gitea.io/gitea/vendor/github.com/unrolled/render/render.go:322 (0x1563782)
        /go/src/code.gitea.io/gitea/vendor/github.com/unrolled/render/render.go:202 (0x1562f8b)
        /go/src/code.gitea.io/gitea/vendor/github.com/unrolled/render/render.go:146 (0x15629d2)
        /go/src/code.gitea.io/gitea/modules/templates/base.go:88 (0x16b0769)
        /go/src/code.gitea.io/gitea/routers/web/base.go:125 (0x225f284)
        /go/src/code.gitea.io/gitea/routers/web/web.go:95 (0x2261284)
        /go/src/code.gitea.io/gitea/routers/init.go:147 (0x22817ba)
        /go/src/code.gitea.io/gitea/cmd/web.go:158 (0x239741a)
        /go/src/code.gitea.io/gitea/vendor/github.com/urfave/cli/app.go:524 (0x1740884)
        /go/src/code.gitea.io/gitea/vendor/github.com/urfave/cli/command.go:173 (0x17415f8)
        /go/src/code.gitea.io/gitea/vendor/github.com/urfave/cli/app.go:277 (0x173e8c7)
        /go/src/code.gitea.io/gitea/main.go:115 (0x23d3e69)
        /usr/local/go/src/runtime/proc.go:225 (0x443995)
        /usr/local/go/src/runtime/asm_amd64.s:1371 (0x47b360)
```

The documentation for the [fallback mail subject](d989247bb0/services/mailer/mail_issue.go (L14-L16)) was missing `{{}}` around `.Issue.Index`.

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lauris BH <lauris@nix.lv>

* Redo #13836 with new shortcode
(https://gitea.com/gitea/theme/pulls/90)

* add Api Usage

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

* align "make help"

* format

* untouch build/generate-svg.js

* untouch .eslintrc

* combine editorconfig's

* rm editorconfig

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

* 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>