Allow user-customisable page on permission denied instead of packaged template

Author: mhindery

djangosaml2/templates/djangosaml2/permission_denied.html

@@ -255,7 +255,7 @@ def assertion_consumer_service(request,
                              create_unknown_user=create_unknown_user)
     if user is None:
         logger.error('The user is None')
-        return render(request, 'djangosaml2/permission_denied.html', status=403)
+        raise PermissionDenied
 
     auth.login(request, user)
     _set_subject_id(request.session, session_info['name_id'])