The `serialize` method of the Notification model now includes the
`created_by_name`. If a notification was sent as a one-off message this
value will now be the name of the person who sent the notification. If
a notification was sent through the API or by a CSV upload we don't
record the id of the sender, so `created_by_name` will be `None`.
This change affects the data that gets returned from these endpoints:
* /v2/notifications/<notification_id>
* /v2/notifications