Revert "remove htmlpurifier"

This reverts commit c21a462d52.

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

@@ -0,0 +1,13 @@
+URI.DisableExternalResources
+TYPE: bool
+VERSION: 1.3.0
+DEFAULT: false
+--DESCRIPTION--
+Disables the embedding of external resources, preventing users from
+embedding things like images from other hosts. This prevents access
+tracking (good for email viewers), bandwidth leeching, cross-site request
+forging, goatse.cx posting, and other nasties, but also results in a loss
+of end-user functionality (they can't directly post a pic they posted from
+Flickr anymore). Use it if you don't have a robust user-content moderation
+team.
+--# vim: et sw=4 sts=4