feat: mark forum response notifications transactional

The user opts into them when making a post / through the forums UI.
They are not truly a marketing email or similar.

lms/djangoapps/discussion/tasks.py

@@ -57,7 +57,9 @@ def update_discussions_map(context):
 
 
 class ResponseNotification(BaseMessageType):
-    pass
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.options['transactional'] = True
 
 
 @shared_task(base=LoggedTask)