Add DVLA callback:

* Process SNS callback, trigger the update notifications celery task
* Put autoconfirm into its own method and use in callbacks

app/notifications/utils.py

@@ -16,3 +16,10 @@ def confirm_subscription(confirmation_request):
         raise e
 
     return confirmation_request['TopicArn']
+
+
+def autoconfirm_subscription(req_json):
+    if req_json.get('Type') == 'SubscriptionConfirmation':
+        current_app.logger.info("SNS subscription confirmation url: {}".format(req_json['SubscribeURL']))
+        subscribed_topic = confirm_subscription(req_json)
+        return subscribed_topic