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.
I think it's more conventional for this project to just execute the body of `addEventHandlers` in the module and just require the module here.