Serialise content in broadcast message response

This will let us show the content of the broadcast message in places we
can’t at the moment.

app/models.py

@@ -2242,6 +2242,12 @@ class BroadcastMessage(db.Model):
     approved_by = db.relationship('User', foreign_keys=[approved_by_id])
     cancelled_by = db.relationship('User', foreign_keys=[cancelled_by_id])
 
+    @property
+    def content(self):
+        return self.template._as_utils_template_with_personalisation(
+            self.personalisation
+        ).content_with_placeholders_filled_in
+
     @property
     def personalisation(self):
         if self._personalisation:
@@ -2261,6 +2267,7 @@ class BroadcastMessage(db.Model):
             'template_id': str(self.template_id),
             'template_version': self.template_version,
             'template_name': self.template.name,
+            'template_content': self.content,
 
             'personalisation': self.personalisation,
             'areas': self.areas.get("areas", []),

tests/app/broadcast_message/test_rest.py

@@ -24,6 +24,8 @@ def test_get_broadcast_message(admin_request, sample_broadcast_service):
     )
 
     assert response['id'] == str(bm.id)
+    assert response['template_id'] == str(t.id)
+    assert response['template_content'] == t.content
     assert response['template_name'] == t.name
     assert response['status'] == BroadcastStatusType.DRAFT
     assert response['created_at'] is not None