Revert "remove htmlpurifier"

This reverts commit c21a462d52.

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

@@ -0,0 +1,17 @@
+URI.AllowedSchemes
+TYPE: lookup
+--DEFAULT--
+array (
+  'http' => true,
+  'https' => true,
+  'mailto' => true,
+  'ftp' => true,
+  'nntp' => true,
+  'news' => true,
+)
+--DESCRIPTION--
+Whitelist that defines the schemes that a URI is allowed to have.  This
+prevents XSS attacks from using pseudo-schemes like javascript or mocha.
+There is also support for the <code>data</code> and <code>file</code>
+URI schemes, but they are not enabled by default.
+--# vim: et sw=4 sts=4