Clear cache when cancelling broadcast via the API

Before we implemented ‘cancel’ any updates to a broadcast went through
the admin app. This meant the admin app could deal with clearing the
cache any time a broadcast was updated by a user performing an action.

Now that a broadcast can be updated without the admin app being involved
we have another place we need to clear the cache from.

If we don’t do this then the broadcast can look like it’s still going
even though it’s successfully been cancelled.

tests/app/v2/broadcast/test_post_broadcast.py

@@ -134,6 +134,8 @@ def test_valid_cancel_broadcast_request_calls_validate_and_update_broadcast_mess
     api_key = create_api_key(service=sample_broadcast_service)
     auth_header = create_service_authorization_header(service_id=sample_broadcast_service.id)
 
+    mock_redis_delete = mocker.patch('app.redis_store.delete')
+
     # create a broadcast
     response_for_create = client.post(
         path='/v2/broadcast',
@@ -165,6 +167,9 @@ def test_valid_cancel_broadcast_request_calls_validate_and_update_broadcast_mess
         expected_status,
         api_key_id=api_key.id
     )
+    mock_redis_delete.assert_called_once_with(
+        f'service-{sample_broadcast_service.id}-broadcast-message-{broadcast_message.id}'
+    )
 
 
 @pytest.mark.parametrize('cap_xml_document, expected_status, expected_error', (