fixes issues history overlapping issues changeset

Formerly the history of an issue would overlap the issue's changeset which would result in the user not beeing able to click on the links in the changeset. The solution is not perfect. It would look much nicer if the changesets would float right. But this is not possible with the content itself having no fixed with.
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
bumps rails version to 2.3.18