mod_pubsub_post: Allow specifying item id by a special mapping If `{http://jabber.org/protocol/pubsub}id` exists as an attribute at the top level then it is used, otherwise the previous behavior (.id if string)

<tr>
  <th>What is {x} {op} {y}?</th><td>
    <input name="captcha_challenge" type="hidden" value="{challenge}">
    <input name="captcha_reply" pattern="[0-9]+" required type="number">
  </td></tr>