Add ability to follow/unfollow groups

To test:
  * Click follow/unfollow buttons on group listing page (http://localhost:8000/group/)
  * Click follow/unfollow buttons on group detail page

The follow button should toggle when clicked and should be persistent across page refreshes.
What I should have said was, I think `Follow.objects. ...` is clearer and doesn't introduce a one vs many ambiguity.