1. Add a `autocomplete/project_groups` controller action. Prefer this to
   `autocomplete/groups` because the latter (wierdly) seems to break
   other dropdowns that list groups.

2. This commit does _not_ change the actual UI, because this is probably
   better saved for @alfredo1 to work on. However, all the other pieces
   have been set up, so there shouldn't be a need for any more backend
   changes.