Add two-way messaging view

> Once an inbound message has been received, there should be a way to
> see the other messages in the system from the same service to the same
> number. Both in and outbound. Nice inbox/whatsapp stylee view or some
> such. This way the context of the reply is understood.
>
> Initially will only see the outbound template, not the actual message,
> but we’re going to change this for the rest (soon), so that you can
> always see the full message for all outbound.

app/notify_client/notification_api_client.py

@@ -54,3 +54,8 @@ class NotificationApiClient(NotifyAdminAPIClient):
                 url='/service/{}/notifications'.format(service_id),
                 params=params
             )
+
+    def get_notification(self, service_id, notification_id):
+        return self.get(
+            url='/service/{}/notifications/{}'.format(service_id, notification_id)
+        )