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.

tests/app/clients/test_reach.py

@@ -0,0 +1 @@
+# TODO: all of the tests