Fixed #1403 -- 'Add' green plus sign no longer appears in admin change forms...

Fixed #1403 -- 'Add' green plus sign no longer appears in admin change forms for fields whose related models don't have an admin

git-svn-id: http://code.djangoproject.com/svn/django/trunk@2991 bcc190cf-cafb-0310-a4f2-bffc1f526a37