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/conftest.py

@@ -351,7 +351,8 @@ def sample_notification(notify_db,
         'reference': reference,
         'created_at': created_at,
         'content_char_count': content_char_count,
-        'personalisation': personalisation
+        'personalisation': personalisation,
+        'notification_type': template.template_type
     }
     if job_row_number:
         data['job_row_number'] = job_row_number