⬆️ 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/composer/autoload_namespaces.php

@@ -8,4 +8,5 @@ $baseDir = dirname($vendorDir);
 return array(
     'OAuth2' => array($vendorDir . '/bshaffer/oauth2-server-php/src'),
     'Michelf' => array($vendorDir . '/michelf/php-markdown'),
+    'HTMLPurifier' => array($vendorDir . '/ezyang/htmlpurifier/library'),
 );