Revert "remove htmlpurifier"

This reverts commit c21a462d52.

lib/htmlpurifier/library/HTMLPurifier/ConfigSchema/schema/CSS.AllowTricky.txt

@@ -0,0 +1,11 @@
+CSS.AllowTricky
+TYPE: bool
+DEFAULT: false
+VERSION: 3.1.0
+--DESCRIPTION--
+This parameter determines whether or not to allow "tricky" CSS properties and
+values. Tricky CSS properties/values can drastically modify page layout or
+be used for deceptive practices but do not directly constitute a security risk.
+For example, <code>display:none;</code> is considered a tricky property that
+will only be allowed if this directive is set to true.
+--# vim: et sw=4 sts=4