Add boilerplate for Reach SMS callbacks

This is enough to update a notification in DB:

1. First create a notification in the UI and sent it.

2. Then reset its attributes to pretend it's for Reach.

    update notifications set
      sent_at = null,
      sent_by = null,
      notification_status='sending'
    where id='some-uuid';

3. Change "notification_id" to "<some-uuid>" in the code.

4. Call the boilerplate endpoint for Reach callbacks.

    curl -X POST localhost:6011/notifications/sms/reach

Interestingly there's no foreign key constraint on "sent_by" in the
DB, so this just works: the notification is updated.

app/notifications/notifications_sms_callback.py

@@ -54,6 +54,28 @@ def process_firetext_response():
     return jsonify(result='success'), 200
 
 
+@sms_callback_blueprint.route('/reach', methods=['POST'])
+def process_reach_response():
+    client_name = 'Reach'
+
+    # TODO: validate request
+    errors = None
+
+    if errors:
+        raise InvalidRequest(errors, status_code=400)
+
+    status = 'TODO-d'  # TODO
+    detailed_status_code = 'something'  # TODO
+    provider_reference = 'notification_id'  # TODO
+
+    process_sms_client_response.apply_async(
+        [status, provider_reference, client_name, detailed_status_code],
+        queue=QueueNames.SMS_CALLBACKS,
+    )
+
+    return jsonify(result='success'), 200
+
+
 def validate_callback_data(data, fields, client_name):
     errors = []
     for f in fields: