Save and view issue/comment content history (#16909)

Save and view issue/comment content history (#16909)

* issue content history

* Use timeutil.TimeStampNow() for content history time instead of issue/comment.UpdatedUnix (which are not updated in time)

* i18n for frontend

* refactor

* clean up

* fix refactor

* re-format

* temp refactor

* follow db refactor

* rename IssueContentHistory to ContentHistory, remove empty model tags

* fix html

* use avatar refactor to generate avatar url

* add unit test, keep at most 20 history revisions.

* re-format

* syntax nit

* Add issue content history table

* Update models/migrations/v197.go

Co-authored-by: 6543 <6543@obermui.de>
* fix merge

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lauris BH <lauris@nix.lv>