Install gems with user `git`, not `gitlab`

Fixed JSON description of system hook

- remove admin kaminari theme. Use gitlab theme everywhere
- use bootstrap styled for gitlab admin themes
- dont reload page when change issue filter

Fixed JSON description of system hook to accurately describe what is actually being sent.

Merge branch 'coherent-and-readable-starting-instructions' of /home/git/repositories/gitlab/gitlabhq

Update user_team_manager.rb