Many thanks to bendavis78 for the initial patch, and for input from others.

Also fixed #7309. If people were relying on the undocumented default ordering
applied by the admin before, they will need to add 'ordering = ["-pk"]' to
their ModelAdmin.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16316 bcc190cf-cafb-0310-a4f2-bffc1f526a37