Fixed #10505: added support for bulk admin actions, including a globally-available "delete selected" action. See the documentation for details.

This work started life as Brian Beck's "django-batchadmin." It was rewritten for inclusion in Django by Alex Gaynor, Jannis Leidel (jezdez), and Martin Mahner (bartTC). Thanks, guys!

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10121 bcc190cf-cafb-0310-a4f2-bffc1f526a37