Revert "remove htmlpurifier"

This reverts commit c21a462d52.

lib/htmlpurifier/library/HTMLPurifier/ConfigSchema/schema/Cache.DefinitionImpl.txt

@@ -0,0 +1,14 @@
+Cache.DefinitionImpl
+TYPE: string/null
+VERSION: 2.0.0
+DEFAULT: 'Serializer'
+--DESCRIPTION--
+
+This directive defines which method to use when caching definitions,
+the complex data-type that makes HTML Purifier tick. Set to null
+to disable caching (not recommended, as you will see a definite
+performance degradation).
+
+--ALIASES--
+Core.DefinitionCache
+--# vim: et sw=4 sts=4