* remove unnecessary web context data fields, and unify the i18n/translation related functions to `Locale`
* in development, show an error if a translation key is missing
* remove the unnecessary loops `for _, lang := range translation.AllLangs()` for every request, which improves the performance slightly
* use `ctx.Locale.Language()` instead of `ctx.Data["Lang"].(string)`
* add more comments about how the Locale/LangType fields are used
* Improve: make diff result better, make the HTML element fit the full height in the content history diff dialog
* Bug fix: when edit the main issue, the poster is wrongly set to the issue poster
* 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>
