Fix an XSS vulnerability

Updated the viewthread.php file to fix a stored XSS vulnerability in the title of forum posts.

Author: HexPandaa

main/forum/viewthread.php

@@ -634,7 +634,7 @@
  'Aw:',
  ];
  $replace = '<span>'.Display::returnFontAwesomeIcon('mail-reply').'</span>';
- $post['post_title'] = str_replace($search, $replace, $post['post_title']);
+ $post['post_title'] = str_replace($search, $replace, Security::remove_XSS($post['post_title']));
 
  // The post title
  $titlePost = Display::tag('h3', $post['post_title'], ['class' => 'forum_post_title']);