Revert "remove htmlpurifier"

This reverts commit c21a462d52.

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

@@ -0,0 +1,9 @@
+URI.OverrideAllowedSchemes
+TYPE: bool
+DEFAULT: true
+--DESCRIPTION--
+If this is set to true (which it is by default), you can override
+%URI.AllowedSchemes by simply registering a HTMLPurifier_URIScheme to the
+registry.  If false, you will also have to update that directive in order
+to add more schemes.
+--# vim: et sw=4 sts=4