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/celery/broadcast_message_tasks.py

@@ -0,0 +1,35 @@
+import requests
+from flask import current_app
+from notifications_utils.statsd_decorators import statsd
+
+from app import notify_celery
+from app.schemas import template_schema
+
+
+from app.dao.broadcast_message_dao import dao_get_broadcast_message_by_id
+
+
+@notify_celery.task(name="send-broadcast-message")
+@statsd(namespace="tasks")
+def send_broadcast_message(broadcast_message_id, provider='stub-1'):
+    broadcast_message = dao_get_broadcast_message_by_id(broadcast_message_id)
+
+    current_app.logger.info(
+        f'sending broadcast_message {broadcast_message_id} '
+        f'status {broadcast_message.status} to {provider}'
+    )
+
+    payload = {
+        "template": template_schema.dump(broadcast_message.template).data,
+        "broadcast_message": broadcast_message.serialize(),
+    }
+    resp = requests.post(
+        f'{current_app.config["CBC_PROXY_URL"]}/broadcasts/{provider}',
+        json=payload
+    )
+    resp.raise_for_status()
+
+    current_app.logger.info(
+        f'broadcast_message {broadcast_message.id} '
+        f'status {broadcast_message.status} sent to {provider}'
+    )