addOnDoneRedirect = (oldPath, req) => {
  const separator = oldPath.indexOf("?") > -1 ? "&" : "?";
  if (req.query.on_done_redirect) {
    return `${oldPath}${separator}on_done_redirect=${req.query.on_done_redirect}`;
  }
  return oldPath;
}
appends 'req.query.on_done_redirect' to 'oldPath' if it exists
@param {string} oldPath path without 'on_done_redirect'
@param {any} req express request
@returns a new string with or without on_done_redirect=...