Admin can make other users admin via the Users page
Related to #450 (closed).
Currently this is only enabled for NCRC. It should be enabled for all instance types. An admin user should be able to promote another user to admin status or demote them to non-admin.
NOTE: I think it's important that an admin user should not be able to demote themselves, as otherwise we could accidentally end up with an instance without any admin users at all, and no way to recover except by directly accessing the DB. We should check whether the current implementation allows this, and if so we need to make sure it doesn't.