add redact to notification with template schema.

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

app/models.py

@@ -443,6 +443,8 @@ class Template(db.Model):
         default=NORMAL
     )
 
+    redact_personalisation = association_proxy('template_redacted', 'redact_personalisation')
+
     def get_link(self):
         # TODO: use "/v2/" route once available
         return url_for(