Fix comments

Fixes:

* drag-n-drop works for diff comments
* selecting images works for diff comments
* preview works for edit comments
* drag-n-drop works for edit comments
* selecting images works for edit comments
* if comment presented on 'Discussion' and 'Changes' tabs - after edit it updates on both tabs.
* Correctly hide form when edit comment on diff
* fix full-screen for diff comments

See merge request !1398