Fixed #8498 -- Add the ability to customize the template for app_index similar...

Fixed #8498 -- Add the ability to customize the template for app_index similar to the other views on the AdminSite class. Thanks Jannis Leidel for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8487 bcc190cf-cafb-0310-a4f2-bffc1f526a37