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/notifications/notifications_letter_callback.py

@@ -0,0 +1,39 @@
+from datetime import datetime
+
+from flask import (
+    Blueprint,
+    jsonify,
+    request,
+    current_app,
+    json
+)
+
+from app import statsd_client
+from app.clients.email.aws_ses import get_aws_responses
+from app.dao import (
+    notifications_dao
+)
+
+from app.notifications.process_client_response import validate_callback_data
+
+letter_callback_blueprint = Blueprint('notifications_letter_callback', __name__)
+
+from app.errors import (
+    register_errors,
+    InvalidRequest
+)
+
+register_errors(letter_callback_blueprint)
+
+
+@letter_callback_blueprint.route('/notifications/letter/dvla', methods=['POST'])
+def process_letter_response():
+    try:
+        dvla_request = json.loads(request.data)
+        current_app.logger.info(dvla_request)
+        return jsonify(
+            result="success", message="DVLA callback succeeded"
+        ), 200
+    except ValueError:
+        error = "DVLA callback failed: invalid json"
+        raise InvalidRequest(error, status_code=400)