course admin team: handle is_staff users

A user with `is_staff=True` is treated as being in all groups. This is problematic
when we care about the user's staff/instructor role for a course: you can't remove
the instructor role. This commit changes the `is_user_in_course_group_role` function
to allow the caller to specify that it should not check the `is_staff` attribute
on the user.