deactivate user. PMT #95450

A superuser can now deactivate a user account.

They are prompted with a form to reassign items and projects
that were owned/assigned/caretaker'ed by the user.
I know we're using `get_absolute_url()` in a lot of places in dmt, but I would like to start relying instead on the django router for this logic, and do something like `reverse('user_detail', args=(u.username,))` here. It's not a pressing issue though, and let me know if there are advantages to `get_absolute_url` that you'd like to keep.