Add broadcasts worker and queue

This worker will be responsible for handing all broadcasts tasks.

It is based on the internal worker which is currently handling broadcast
tasks.

Concurrency of 2 has been chosen fairly arbitrarily. Gunicorn will be
running 4 worker processes so we will end up with the ability to process
8 tasks per app instance given this.

tests/app/test_config.py

@@ -60,7 +60,7 @@ def test_load_config_if_cloudfoundry_not_available(reload_config):
 def test_queue_names_all_queues_correct():
     # Need to ensure that all_queues() only returns queue names used in API
     queues = QueueNames.all_queues()
-    assert len(queues) == 16
+    assert len(queues) == 17
     assert set([
         QueueNames.PRIORITY,
         QueueNames.PERIODIC,
@@ -77,5 +77,6 @@ def test_queue_names_all_queues_correct():
         QueueNames.LETTERS,
         QueueNames.SMS_CALLBACKS,
         QueueNames.SAVE_API_EMAIL,
-        QueueNames.SAVE_API_SMS
+        QueueNames.SAVE_API_SMS,
+        QueueNames.BROADCASTS,
     ]) == set(queues)