Bug 937673 - Modify groups model to allow user-created curated groups

* Add flags for functional area, members_can_leave, visible
* Get rid of system, auto_complete, and related fields

- We can probably squash the migrations down to 3 or so, but it'll be tedious so I'll do that when we've agreed the logic is final.

- Also addresses https://bugzilla.mozilla.org/show_bug.cgi?id=921147