Revert "remove htmlpurifier"

This reverts commit c21a462d52.

lib/htmlpurifier/library/HTMLPurifier/ConfigSchema/schema/URI.MungeResources.txt

@@ -0,0 +1,17 @@
+URI.MungeResources
+TYPE: bool
+VERSION: 3.1.1
+DEFAULT: false
+--DESCRIPTION--
+<p>
+    If true, any URI munging directives like %URI.Munge
+    will also apply to embedded resources, such as <code>&lt;img src=""&gt;</code>.
+    Be careful enabling this directive if you have a redirector script
+    that does not use the <code>Location</code> HTTP header; all of your images
+    and other embedded resources will break.
+</p>
+<p>
+    <strong>Warning:</strong> It is strongly advised you use this in conjunction
+    %URI.MungeSecretKey to mitigate the security risk of an open redirector.
+</p>
+--# vim: et sw=4 sts=4