Use service model to look up service attributes

This is better than just keying into the JSON because it means you get
an exception straight away when looking up a key that doesn’t exist
(which via mocking you could ordinarily miss).

app/template_previews.py

@@ -11,7 +11,7 @@ class TemplatePreview:
             'letter_contact_block': template.get('reply_to_text', ''),
             'template': template,
             'values': values,
-            'dvla_org_id': current_service['dvla_organisation'],
+            'dvla_org_id': current_service.dvla_organisation,
         }
         resp = requests.post(
             '{}/preview.{}{}'.format(