Pass letter logo filename to template preview

We now pass `filename`, the filename of the letter logo to use, through
to Template Preview in addition to the `dvla_org_id`. Once Template
Preview has been updated to only use the `filename` we will stop
sending the `dvla_org_id`.

app/template/rest.py

@@ -247,6 +247,7 @@ def preview_letter_template_by_notification_id(service_id, notification_id, file
             'template': template_for_letter_print,
             'values': notification.personalisation,
             'date': notification.created_at.isoformat(),
+            'filename': service.dvla_organisation.filename,
             'dvla_org_id': service.dvla_organisation_id,
         }