Ability to assign issues to groups (#2964).

Option is disabled by default. It can be turned on in application settings.
Include issues asigned to user's groups when using 'assigned to me' filter (#2964).
renames variable invalidated by cherry-picking commit 578fdc62f26c23951b2d2c2b9be0040c7ade0634
fixes filtering by watchers broken by 9f18636