This now follows the logic of the superuser, and doesn't respect the ability for a teacher to be sandboxed to a group.

I would suggest one of two changes:
* Allow teachers to be in a group
or
* Make the if statement with superuser to also check for teacher status.
* Assert in FacilityUser.save() that if is_teacher, then group is None.
* (re-) add code to add_facility_user.html to NOT show choosing a group, if adding a teacher (which needs to be passed as a flag from the view)