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.

app/config.py

@@ -33,6 +33,7 @@ class QueueNames(object):
     SANITISE_LETTERS = 'sanitise-letter-tasks'
     SAVE_API_EMAIL = 'save-api-email-tasks'
     SAVE_API_SMS = 'save-api-sms-tasks'
+    BROADCASTS = 'broadcast-tasks'
 
     @staticmethod
     def all_queues():
@@ -52,7 +53,8 @@ class QueueNames(object):
             QueueNames.LETTERS,
             QueueNames.SMS_CALLBACKS,
             QueueNames.SAVE_API_EMAIL,
-            QueueNames.SAVE_API_SMS
+            QueueNames.SAVE_API_SMS,
+            QueueNames.BROADCASTS,
         ]