MDL-31785 import HTMLPurifier 4.4.0+

Includes one extra fix for _blank 8c9d461a6259d7b8ac4ae5c9c9e95ab176cfcda3 compared to 4.4.0 release.

lib/htmlpurifier/HTMLPurifier/Config.php

@@ -20,7 +20,7 @@ class HTMLPurifier_Config
     /**
      * HTML Purifier's version
      */
-    public $version = '4.3.0';
+    public $version = '4.4.0';
 
     /**
      * Bool indicator whether or not to automatically finalize
@@ -44,7 +44,7 @@ class HTMLPurifier_Config
     /**
      * Parser for variables
      */
-    protected $parser;
+    protected $parser = null;
 
     /**
      * Reference HTMLPurifier_ConfigSchema for value checking
@@ -668,7 +668,7 @@ class HTMLPurifier_Config
      */
     public function finalize() {
         $this->finalized = true;
-        unset($this->parser);
+        $this->parser = null;
     }
 
     /**