Auto-subscribe user to repository when they commit/tag to it (#7657)

* Add support for AUTO_WATCH_ON_CHANGES and AUTO_WATCH_ON_CLONE

* Update models/repo_watch.go

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

* Round up changes suggested by lafriks

* Added changes suggested from automated tests

* Updated deleteUser to take RepoWatchModeDont into account, corrected inverted DefaultWatchOnClone and DefaultWatchOnChanges behaviour, updated and added tests.

* Reinsert import "github.com/Unknwon/com" on http.go

* Add migration for new column `watch`.`mode`

* Remove serv code

* Remove WATCH_ON_CLONE; use hooks, add integrations

* Renamed watch_test.go to repo_watch_test.go

* Correct fmt

* Add missing EOL

* Correct name of test function

* Reword cheat and ini descriptions

* Add update to migration to ensure column value

* Clarify comment

Co-Authored-By: zeripath <art27@cantab.net>

* Simplify if condition

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -303,6 +303,7 @@ relation to port exhaustion.
   on this instance.
 - `SHOW_REGISTRATION_BUTTON`: **! DISABLE\_REGISTRATION**: Show Registration Button
 - `AUTO_WATCH_NEW_REPOS`: **true**: Enable this to let all organisation users watch new repos when they are created
+- `AUTO_WATCH_ON_CHANGES`: **false**: Enable this to make users watch a repository after their first commit to it
 - `DEFAULT_ORG_VISIBILITY`: **public**: Set default visibility mode for organisations, either "public", "limited" or "private".
 - `DEFAULT_ORG_MEMBER_VISIBLE`: **false** True will make the membership of the users visible when added to the organisation.