Add TemplateHistory.redact_personalisation attribute

TemplateHistory objects need to be connected to the template's
TemplateRedacted record. This requires setting up a new SQLAlchemy
relationship to allow accessing redact_personalisation from
TemplateHistory instances.

We can avoid creating a foreign key in this case by setting up an
explicit `primaryjoin` expression. Since TemplateHistory.id is
created from Template.id and TemplateRedacted.template_id is
already a foreign key to Template.id the foreign key should always
be valid even without a DB constraint.

app/models.py

@@ -645,6 +645,11 @@ class TemplateHistory(db.Model):
                              nullable=False,
                              default=NORMAL)
 
+    template_redacted = db.relationship('TemplateRedacted', foreign_keys=[id],
+                                        primaryjoin='TemplateRedacted.template_id == TemplateHistory.id')
+
+    redact_personalisation = association_proxy('template_redacted', 'redact_personalisation')
+
     def get_link(self):
         return url_for(
             "v2_template.get_template_by_id",