Better user registration UX on the distributed server 

Three changes; first two were suggested by @wangguan59 , the third objected to by @rtibbles :
* When a user registers or is created by an admin, show a success message.
* When an admin creates a user, redirect them to the same page, rather than the homepage (for easier bulk create)
* Allow a teacher to be added to a group (can be used for initial setting of coach report queries, or later for sandboxing if desired).  By default, no group is selected (nor required), so for most things this is simply a no-op--just increases flexibility (without confusion, I believe)

Now that teachers can be in a group, the FacilityTeacherForm became obsolete, and the logic of add_facility_user became more simple.