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

* `git` -> `Git`
* `Github` and `github` -> `GitHub`
* `crowdin` -> `Crowdin`
* `git-lfs` -> `Git LFS`
* `githooks`, `git hooks`, `git-hooks` -> `Git Hooks`
* `discord` -> `Discord`
* `2fa` -> `2FA`
* `gitlab` and `Gitlab` -> `GitLab`
* `web hook` -> `webhook`
* `linux` -> `Linux`
* `sqlite` -> `SQLite`
* `MYSQL` and `mysql` -> `MySQL`
* rename refs to `master` branch -> `main`
* Fix English grammar

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

* Improve SMTP authentication, Fix user creation bugs and add LDAP cert/key options

This PR has two parts:

Improvements for SMTP authentication:

* Default to use SMTPS if port is 465, and allow setting of force SMTPS.
* Always use STARTTLS if available
* Provide CRAM-MD5 mechanism
* Add options for HELO hostname disabling
* Add options for providing certificates and keys
* Handle application specific password response as a failed user login
instead of as a 500.

Close #16104

Fix creation of new users:

* A bug was introduced when allowing users to change usernames which
prevents the creation of external users.
* The LoginSource refactor also broke this page.

Close #16104

Signed-off-by: Andrew Thornton <art27@cantab.net>

Fix #16639

* Fix various doc, user-facing, and source comment typos

Found via `codespell -q 3 -S ./options/locale,./vendor -L ba,pullrequest,pullrequests,readby`

* Encrypt LDAP bind password in db with SECRET_KEY

The LDAP source bind password are currently stored in plaintext in the db
This PR simply encrypts them with the setting.SECRET_KEY.

Fix #15460

Signed-off-by: Andrew Thornton <art27@cantab.net>

* remove ui warning regarding unencrypted password

Co-authored-by: silverwind <me@silverwind.io>

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>

* Rename custom/conf/app.ini.sample to custom/conf/app.sample.ini for better syntax light on editor

* rename to app.example.ini

* per @6543 's comment, update all references on docs

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

* Remove IE11 support

With master now on 1.13, it's time to drop IE11 for good. The woff
variants are also in use by Opera Mini but it has even less market share
and I can only imagine how broken the UI is in it.

Fixes: https://github.com/go-gitea/gitea/issues/6147

* update docs

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

* Add single sign-on support via SSPI on Windows

* Ensure plugins implement interface

* Ensure plugins implement interface

* Move functions used only by the SSPI auth method to sspi_windows.go

* Field SSPISeparatorReplacement of AuthenticationForm should not be required via binding, as binding will insist the field is non-empty even if another login type is selected

* Fix breaking of oauth authentication on download links. Do not create new session with SSPI authentication on download links.

* Update documentation for the new 'SPNEGO with SSPI' login source

* Mention in documentation that ROOT_URL should contain the FQDN of the server

* Make sure that Contexter is not checking for active login sources when the ORM engine is not initialized (eg. when installing)

* Always initialize and free SSO methods, even if they are not enabled, as a method can be activated while the app is running (from Authentication sources)

* Add option in SSPIConfig for removing of domains from logon names

* Update helper text for StripDomainNames option

* Make sure handleSignIn() is called after a new user object is created by SSPI auth method

* Remove default value from text of form field helper

Co-Authored-By: Lauris BH <lauris@nix.lv>

* Remove default value from text of form field helper

Co-Authored-By: Lauris BH <lauris@nix.lv>

* Remove default value from text of form field helper

Co-Authored-By: Lauris BH <lauris@nix.lv>

* Only make a query to the DB to check if SSPI is enabled on handlers that need that information for templates

* Remove code duplication

* Log errors in ActiveLoginSources

Co-Authored-By: Lauris BH <lauris@nix.lv>

* Revert suffix of randomly generated E-mails for Reverse proxy authentication

Co-Authored-By: Lauris BH <lauris@nix.lv>

* Revert unneeded white-space change in template

Co-Authored-By: Lauris BH <lauris@nix.lv>

* Add copyright comments at the top of new files

* Use loopback name for randomly generated emails

* Add locale tag for the SSPISeparatorReplacement field with proper casing

* Revert casing of SSPISeparatorReplacement field in locale file, moving it up, next to other form fields

* Update docs/content/doc/features/authentication.en-us.md

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

* Remove Priority() method and define the order in which SSO auth methods should be executed in one place

* Log authenticated username only if it's not empty

* Rephrase helper text for automatic creation of users

* Return error if more than one active SSPI auth source is found

* Change newUser() function to return error, letting caller log/handle the error

* Move isPublicResource, isPublicPage and handleSignIn functions outside SSPI auth method to allow other SSO methods to reuse them if needed

* Refactor initialization of the list containing SSO auth methods

* Validate SSPI settings on POST

* Change SSPI to only perform authentication on its own login page, API paths and download links. Leave Toggle middleware to redirect non authenticated users to login page

* Make 'Default language' in SSPI config empty, unless changed by admin

* Show error if admin tries to add a second authentication source of type SSPI

* Simplify declaration of global variable

* Rebuild gitgraph.js on Linux

* Make sure config values containing only whitespace are not accepted

* Clean up spelling, grammar, perspective, whitespace, language, markup, etc.

* import docs into main repository

Signed-off-by: Matti Ranta <matti@mdranta.net>