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 much prefer the form
```javascript
function(el) {
    var btn = $(el).find(dom.followButton);
    // ...
}
```