FIX: Tree View Add New Button Parent ID Broken

In relation to:
https://github.com/silverstripe/silverstripe-cms/issues/855

The %s in https://github.com/silverstripe/silverstripe-cms/blob/f937b95eaf2b74ee75b46ad56a44e9126a927690/templates/Includes/CMSPagesController_ContentToolActions.ss

is encoded by https://github.com/silverstripe/silverstripe-framework/blob/a8c1dd7f535bce772a06c9717b09f2ba882300db/control/Controller.php (Line 576)