Add notification_type to notification table.

It seems like an oversight not to include the notification type in the notifcation.
When updating statistics a query to the template table is required to get the type, this update will mean that query does not have to happen.

tests/app/celery/test_tasks.py

@@ -46,6 +46,7 @@ def _notification_json(template, to, personalisation=None, job_id=None, row_numb
         "template": str(template.id),
         "template_version": template.version,
         "to": to,
+        "notification_type": template.template_type
     }
     if personalisation:
         notification.update({"personalisation": personalisation})