casa_admin can deactivate user account

[compwron]

part of epic #4

What type of user is this for? [volunteer/supervisor/admin/all]
admin

Where does/should this occur?
This should be an addition to the admin dashboard under edit Volunteer.

Description
deactivating a volunteer account means the volunteer can no longer log in. This is not needed for MVP right now but we do need to build it.

related to issues
#22