Fixed #16542 -- Made Raw ID form widgets shipped with the admin app render the...

Fixed #16542 -- Made Raw ID form widgets shipped with the admin app render the related object lookup tool only when the related model is effectively registered with the AdminSite.

Also, converted these widgets to reverse named URLs instead of hard-coded '../../...'-style links, refs #15294.
Thanks Florian Apolloner for the initial patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16578 bcc190cf-cafb-0310-a4f2-bffc1f526a37