Don't send email alerts for task validation messages.

-----------------------
This commit does the following:
- Task validation messages are not sent over emails. They are always saved on database (i.e. These messages are displayed on TM notification pages.)
- Task invalidation messages are always saved on database (i.e Can be viewed on TM notification page). Task invalidation messages will be sent over email if user has enabled `task notification` email.
- Task notification setting (Task validation emails on frontend) will only control if task invalidation message is sent over email)

backend/services/messaging/message_service.py

@@ -194,9 +194,13 @@ def _push_messages(messages):
                 and obj.message_type == MessageType.TASK_COMMENT_NOTIFICATION.value
             ):
                 continue
-            if user.tasks_notifications is False and obj.message_type in (
-                MessageType.VALIDATION_NOTIFICATION.value,
-                MessageType.INVALIDATION_NOTIFICATION.value,
+            # Don't send email alerts for task validation messages
+            if obj.message_type == MessageType.VALIDATION_NOTIFICATION.value:
+                messages_objs.append(obj)
+                continue
+            if (
+                user.tasks_notifications is False
+                and obj.message_type == MessageType.INVALIDATION_NOTIFICATION.value
             ):
                 messages_objs.append(obj)
                 continue