Update Notification schema to preserve template_history attribute
Update Notification schema to preserve template_history attribute

`Notification.template_history` relationship has been removed but
we want to keep the `template_history` key in existing notification
serializations, so we serialize it from `Notifications.template`.

This keeps the data format the same, but both `template` and
`template_history` keys will now contain data from the `TemplateHistory`
instance.

alphagov/notifications-api,alphagov/notifications-api