Fixed #8522 -- Allow app_index to take extra_context to be consistent with the...

Fixed #8522 -- Allow app_index to take extra_context to be consistent with the other views in the admin. Thanks Jannis Leidel for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8529 bcc190cf-cafb-0310-a4f2-bffc1f526a37