Show letter preview once file is uploaded

This shows the sanitised letter preview if the file had no validation
errors or the preview with the overlay if it failed validation.

app/notify_client/service_api_client.py

@@ -261,6 +261,12 @@ class ServiceAPIClient(NotifyAdminAPIClient):
         )
         return self.get(endpoint)
 
+    def get_precompiled_template(self, service_id):
+        """
+        Returns the precompiled template for a service, creating it if it doesn't already exist
+        """
+        return self.get('/service/{}/template/precompiled'.format(service_id))
+
     @cache.set('service-{service_id}-templates')
     def get_service_templates(self, service_id):
         """