mod_pubsub_summary: Use pre-escaped UTF-8 sequence for compat This silences a warning about using the non-existent global 'utf8' each time this line was hit. The utf8 library was introduced in Lua 5.3, so the warning would appear with 5.1 and 5.2.

<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>