Wired in a simple callback to handle SNS notifications from S3

S3 will send a message when a file lands - which will trigger processing of DVLA responses.

app/__init__.py

@@ -95,6 +95,7 @@ def register_blueprint(application):
     from app.notifications.receive_notifications import receive_notifications_blueprint
     from app.notifications.notifications_ses_callback import ses_callback_blueprint
     from app.notifications.notifications_sms_callback import sms_callback_blueprint
+    from app.notifications.notifications_letter_callback import letter_callback_blueprint
     from app.authentication.auth import requires_admin_auth, requires_auth, requires_no_auth
     from app.letters.send_letter_jobs import letter_job
 
@@ -155,6 +156,9 @@ def register_blueprint(application):
     letter_job.before_request(requires_admin_auth)
     application.register_blueprint(letter_job)
 
+    letter_callback_blueprint.before_request(requires_no_auth)
+    application.register_blueprint(letter_callback_blueprint)
+
 
 def register_v2_blueprints(application):
     from app.v2.notifications.post_notifications import v2_notification_blueprint as post_notifications