Show personalisation in individual letters

Adds a new endpoint that works like view template/view preview of
letter, so that this page works the same way it does for emails/text
messages (ie showing the full content of the message, including
personalisation).

We’re not worrying about redaction in letters for now.

tests/conftest.py

@@ -1703,6 +1703,7 @@ def mock_get_notification(
             service_id,
             '5407f4db-51c7-4150-8758-35412d42186a',
             content='hello ((name))',
+            subject='blah',
             redact_personalisation=redact_personalisation,
             type_=template_type,
         )