Add a new endpoint for managing a folder, use it to rename folder

- Add a GET / POST view: manage template folder
- Add a template for that view, where a user can rename their
folder
- Add an API client method for updating a folder

- Test the new feature, including the test that service without
permissions cannot manage a folder

app/notify_client/template_folder_api_client.py

@@ -43,5 +43,11 @@ class TemplateFolderAPIClient(NotifyAdminAPIClient):
             template_ids,
         ))
 
+    def update_template_folder(self, service_id, template_folder_id, name):
+        self.post(
+            '/service/{}/template-folder/{}'.format(service_id, template_folder_id),
+            {"name": name}
+        )
+
 
 template_folder_api_client = TemplateFolderAPIClient()