Create top level group as department in tests

- This is in preparation for preventing users
editing the department level group
Remove unused controller behaviour

- We no longer add people on the group edit page
so there's no need to load a list of assignable
people.

- While groups are still added on the edit person
page, it will be done with the org browser so we
are not concerned with filtering a list of groups
and can then remove the set_groups method from
the people_controller.