User management tweaks.

(Resending this because I rebased and github got confused [or I got confused].)

* Fix some permission checks.
* Tweak template language depending whether account will be deleted or deactivated.
* Tweak printing of vesting user's registrar/library.
* raise Http404 instead of redirect if user fails permission check when trying to edit another user -- otherwise we're leaking information about whether a given user account exists.