So that when the admin gets notifications, the template they return
also has a "redact_personalisation" boolean attached to it. Note, it
won't do the redacting on the api - that'll be part of the admin.
Under the hood, this uses an association_proxy, which is essentially
black magic. But it proxies the `redact_personalisation` property of
`TemplateRedacted` onto the `Template` object, so that Marshmallow
can pick it up.
Note: NOT currently added to NotificationWithTemplateHistory