mod_auth_oauth_external: Allow different username in PLAIN vs final JID Mastodon for example having email addresses usernames in login, but a different username in the service itself. Thanks to @tcit@social.tcit.fr for the pointer to a usable validation endpoint for Mastodon, allowing this to be tested.

<tr>
  <td colspan="2">
    <script src="https://hcaptcha.com/1/api.js" async defer></script>
    <div class="h-captcha" data-sitekey="{captcha_public_key}"></div>
  </td>
</tr>