Revert "remove htmlpurifier"

This reverts commit c21a462d52.

lib/htmlpurifier/library/HTMLPurifier/ConfigSchema/schema/Attr.ClassUseCDATA.txt

@@ -0,0 +1,19 @@
+Attr.ClassUseCDATA
+TYPE: bool/null
+DEFAULT: null
+VERSION: 4.0.0
+--DESCRIPTION--
+If null, class will auto-detect the doctype and, if matching XHTML 1.1 or
+XHTML 2.0, will use the restrictive NMTOKENS specification of class. Otherwise,
+it will use a relaxed CDATA definition.  If true, the relaxed CDATA definition
+is forced; if false, the NMTOKENS definition is forced.  To get behavior
+of HTML Purifier prior to 4.0.0, set this directive to false.
+
+Some rational behind the auto-detection:
+in previous versions of HTML Purifier, it was assumed that the form of
+class was NMTOKENS, as specified by the XHTML Modularization (representing
+XHTML 1.1 and XHTML 2.0).  The DTDs for HTML 4.01 and XHTML 1.0, however
+specify class as CDATA.  HTML 5 effectively defines it as CDATA, but
+with the additional constraint that each name should be unique (this is not
+explicitly outlined in previous specifications).
+--# vim: et sw=4 sts=4