User can leave group from group page.
Created by: cirosantilli
Fixes ACCEPTING MERGE/PULL REQUESTS at http://feedback.gitlab.com/forums/176466-general/suggestions/5323575-allow-people-to-leave-a-group
Screenshot:
Implementation notes
Moved some of the "who can remove who from a group" logic from templates into new functions of users_groups model.
Tests:
- named the user and the groups. This led to mass refactoring of the group tests, but what was tested in all the existing tests was kept.
- merged create group, which had a single test, into the group tests.
- moved group test out of group directory since there is a single file now.