🩹 fix: parsing logic for GuestUser to correctly handle Gravatar URLs

[r74tech]

Updated user_parse function to correctly parse GuestUser elements by detecting Gravatar URLs.
If the element contains an image with a Gravatar URL, it is parsed as a GuestUser, and the Gravatar URL is set as the avatar_url attribute.

Wikidot uses Gravatar to display avatars for guest users, as described in the official documentation:
https://www.wikidot.com/more:explore-features#toc13

Examples of GuestUser:

1. http://scp-jp.wikidot.com/forum/t-2268977/

<span class="printuser avatarhover">
  <a href="javascript:;">
    <img class="small" src="http://www.gravatar.com/avatar.php?gravatar_id=23463b99b62a72f26ed677cc556c44e8&default=http://www.wikidot.com/common--images/avatars/default/a16.png&size=16" alt="">
  </a>guest (ゲスト)
</span>

2. http://wikirb.wikidot.com/forum/t-16957468/guest2#post-6679087 (when user icons are disabled)

<span class="printuser avatarhover">
  <a href="javascript:;">
    <img class="small" src="http://www.gravatar.com/avatar.php?gravatar_id=55502f40dc8b7c769880b10874abc9d0&default=http://www.wikidot.com/common--images/avatars/default/a16.png&size=16" alt="">
  </a>guest2 (guest)
</span>

In both examples, the presence of a Gravatar URL in the img tag is used to identify the user as a GuestUser.