Return responses from TaskState.on_action

This allows workflow actions to redirect somewhere for the review to
take place.

wagtail/admin/views/pages.py

@@ -1238,7 +1238,9 @@ def workflow_action(request, page_id):
     if action_name not in set(action[0] for action in actions):
         raise PermissionDenied
 
-    task.on_action(task_state, request.user, action_name)
+    response = task.on_action(task_state, request.user, action_name)
+    if response:
+        return response
 
     return redirect(redirect_to)