MDL-31785 import HTMLPurifier 4.4.0+

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

lib/htmlpurifier/HTMLPurifier/HTMLModule/Forms.php

@@ -35,7 +35,7 @@ class HTMLPurifier_HTMLModule_Forms extends HTMLPurifier_HTMLModule
             'name' => 'CDATA',
             'readonly' => 'Bool#readonly',
             'size' => 'Number',
-            'src' => 'URI#embeds',
+            'src' => 'URI#embedded',
             'tabindex' => 'Number',
             'type' => 'Enum#text,password,checkbox,button,radio,submit,reset,file,hidden,image',
             'value' => 'CDATA',
@@ -84,7 +84,8 @@ class HTMLPurifier_HTMLModule_Forms extends HTMLPurifier_HTMLModule
         $button->excludes = $this->makeLookup(
             'form', 'fieldset', // Form
             'input', 'select', 'textarea', 'label', 'button', // Formctrl
-            'a' // as per HTML 4.01 spec, this is omitted by modularization
+            'a', // as per HTML 4.01 spec, this is omitted by modularization
+            'isindex', 'iframe' // legacy items
         );
 
         // Extra exclusion: img usemap="" is not permitted within this element.