Fixed #7982 -- Corrected ModelAdmin url dispatching to ensure it matching exactly what it needs and doesn't stomp on primary key space. 'add' is a lost cause for now. This originated from #6470. Thanks jdetaeye for the original patch and basith for providing test cases.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8704 bcc190cf-cafb-0310-a4f2-bffc1f526a37