Make the template preview work

The template was displaying raw, eg ‘Hello ((name))’

This commit changes it to use the `.formatted_as_markup` property so the
template is rendered with:
- the placeholders as blue lozenges
- the service name prefixing the message

tests/app/main/views/test_jobs.py

@@ -41,4 +41,6 @@ def test_should_show_page_for_one_job(app_,
             response = client.get(url_for('main.view_job', service_id=service_id, job_id=job_id))
 
         assert response.status_code == 200
-        assert file_name in response.get_data(as_text=True)
+        content = response.get_data(as_text=True)
+        assert "Test Service: Your vehicle tax is about to expire" in content
+        assert file_name in content