Allow admins to revoke admin role

:fork_and_knife: Implement the ability for admins to revoke other admins of their admin role. Also change the way the admin role is granted so it doesn't overwrite all of the users roles it just adds the admin role to the user.
This is more or less what the authorizer is doing, right? Would it make sense to expand the authorizer to also check that `record.is?(:admin)`? Then we could not only reuse the authorizer logic but also only display this button when both users are admins.