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

tests/app/celery/test_broadcast_message_tasks.py

@@ -0,0 +1,61 @@
+import pytest
+import requests_mock
+from requests import RequestException
+
+from app.dao.templates_dao import dao_update_template
+from app.models import BROADCAST_TYPE, BroadcastStatusType
+from app.celery.broadcast_message_tasks import send_broadcast_message
+from tests.app.db import create_template, create_broadcast_message
+
+
+def test_send_broadcast_message_sends_data_correctly(sample_service):
+    t = create_template(sample_service, BROADCAST_TYPE)
+    bm = create_broadcast_message(t, areas=['london'], status=BroadcastStatusType.BROADCASTING)
+
+    with requests_mock.Mocker() as request_mock:
+        request_mock.post("http://test-cbc-proxy/broadcasts/stub-1", json={'valid': 'true'}, status_code=200)
+        send_broadcast_message(broadcast_message_id=str(bm.id))
+
+    assert request_mock.call_count == 1
+    assert request_mock.request_history[0].method == 'POST'
+    assert request_mock.request_history[0].headers["Content-type"] == "application/json"
+
+    cbc_json = request_mock.request_history[0].json()
+    assert cbc_json['template']['id'] == str(t.id)
+    assert cbc_json['template']['template_type'] == BROADCAST_TYPE
+    assert cbc_json['broadcast_message']['areas'] == ['london']
+
+
+def test_send_broadcast_message_sends_old_version_of_template(sample_service):
+    t = create_template(sample_service, BROADCAST_TYPE, content='first content')
+    bm = create_broadcast_message(t, areas=['london'], status=BroadcastStatusType.BROADCASTING)
+
+    t.content = 'second content'
+    dao_update_template(t)
+    assert t.version == 2
+
+    with requests_mock.Mocker() as request_mock:
+        request_mock.post("http://test-cbc-proxy/broadcasts/stub-1", json={'valid': 'true'}, status_code=200)
+        send_broadcast_message(broadcast_message_id=str(bm.id))
+
+    assert request_mock.call_count == 1
+    assert request_mock.request_history[0].method == 'POST'
+    assert request_mock.request_history[0].headers["Content-type"] == "application/json"
+
+    cbc_json = request_mock.request_history[0].json()
+    assert cbc_json['template']['id'] == str(t.id)
+    assert cbc_json['template']['version'] == 1
+    assert cbc_json['template']['content'] == 'first content'
+
+
+def test_send_broadcast_message_errors(sample_service):
+    t = create_template(sample_service, BROADCAST_TYPE)
+    bm = create_broadcast_message(t, status=BroadcastStatusType.BROADCASTING)
+
+    with requests_mock.Mocker() as request_mock:
+        request_mock.post("http://test-cbc-proxy/broadcasts/stub-1", text='503 bad gateway', status_code=503)
+        # we're not retrying or anything for the moment - but this'll ensure any exception gets logged
+        with pytest.raises(RequestException) as ex:
+            send_broadcast_message(broadcast_message_id=str(bm.id))
+
+    assert ex.value.response.status_code == 503