Admin users/superusers can remove their own admin status locking themselves out in future

[pnilan]

Issue

An administrative user or superuser can deselect "administrative" from the edit user page for their own user account. This won't have an immediate effect, but in future login attempts, the user will be locked out of admin dashboard.

Notes

More of a policy issue -- should we allow users to edit their own admin status? Requires further discussion.