Allow broadcasts to be cancelled

Currently this is a `get` request from the dashboard. Once we have a page
for viewing an individual broadcast it should probably show there
instead and be a `get`/confirm/`post` loop like for deleting a template.

app/models/broadcast_message.py

@@ -125,6 +125,13 @@ class BroadcastMessage(JSONModel):
             service_id=self.service_id,
         )
 
+    def cancel_broadcast(self):
+        broadcast_message_api_client.update_broadcast_message_status(
+            'cancelled',
+            broadcast_message_id=self.id,
+            service_id=self.service_id,
+        )
+
 
 class BroadcastMessages(ModelList):