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.
Interesting. I haven't seen us use a comment like this before. Seems like a great idea, barring the usual problem with comments - if this comes to expect more variables, it becomes outdated. It's also 38% of the total size of the file.