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/main/__init__.py

@@ -27,5 +27,6 @@ from app.main.views import (
     feedback,
     providers,
     platform_admin,
-    letter_jobs
+    letter_jobs,
+    conversation,
 )