add send_broadcast_message task

task takes a brodcast_message_id, and makes a post to the cbc-proxy
for now, hardcode the url to the notify stub. the stub requires template
as the admin/api get it, so use the marshmallow schema to json dump it.
Note - this also required us to tweak the BroadcastMessage.serialize
function so that it converts uuids in to ids - flask's jsonify function
does that for free but requests.post doesn't sadly.

if the request fails (either 4xx or 5xx) just raise an exception and let
it bubble up for now - in the future we'll add retry logic

app/dao/broadcast_message_dao.py

@@ -20,6 +20,10 @@ def dao_get_broadcast_message_by_id_and_service_id(broadcast_message_id, service
     ).one()
 
 
+def dao_get_broadcast_message_by_id(broadcast_message_id):
+    return BroadcastMessage.query.get(broadcast_message_id)
+
+
 def dao_get_broadcast_messages_for_service(service_id):
     return BroadcastMessage.query.filter(
         BroadcastMessage.service_id == service_id