getRandomPage = (pageGroup, req) => {
  if (pageGroup.members.length === 0)
    return req.__("Pagegroup %s has no members", pageGroup.name);
  const hash = crypto.createHash("sha1").update(req.sessionID).digest("hex");
  const idx =
    parseInt(hash.substring(hash.length - 4), 16) % pageGroup.members.length;
  const sessionMember = pageGroup.members[idx];
  return Page.findOne({ id: sessionMember.page_id });
}
@param {PageGroup} pageGroup
@param {any} req
@returns the page, null or an error msg