Add checks to check for attempted duplicate additions of things in admin panel.

This code should prevent any nasty 500 errors resulting from attempts to add duplicate badges/assertions/people/issuers/authorizations via the admin panel.
I'm not quite sure this will work, since the ``badge-name`` coming in is something like ``"The Fancy Badge"`` whereas the id expected by ``request.db.badge_exists`` is something like ``"the-fancy-badge"``.