⬆️ Update HTML Purifier library.

Updated HTML Purifier from 4.6.0 to 4.9.2 with better PHP7 compatibility.
Used composer to manage this library.

vendor/ezyang/htmlpurifier/library/HTMLPurifier.composer.php

@@ -0,0 +1,4 @@
+<?php
+if (!defined('HTMLPURIFIER_PREFIX')) {
+    define('HTMLPURIFIER_PREFIX', dirname(__FILE__));
+}