Fixed #11949 -- Added a hook to allow ModelAdmin customization of the delete...

Fixed #11949 -- Added a hook to allow ModelAdmin customization of the delete selected template. Thanks to bendavis78 for the report and patch, and Ramiro Morales for his cleanup work.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12916 bcc190cf-cafb-0310-a4f2-bffc1f526a37