Revert "remove htmlpurifier"

This reverts commit c21a462d52.

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

@@ -0,0 +1,19 @@
+URI.Host
+TYPE: string/null
+VERSION: 1.2.0
+DEFAULT: NULL
+--DESCRIPTION--
+
+<p>
+    Defines the domain name of the server, so we can determine whether or
+    an absolute URI is from your website or not.  Not strictly necessary,
+    as users should be using relative URIs to reference resources on your
+    website.  It will, however, let you use absolute URIs to link to
+    subdomains of the domain you post here: i.e. example.com will allow
+    sub.example.com.  However, higher up domains will still be excluded:
+    if you set %URI.Host to sub.example.com, example.com will be blocked.
+    <strong>Note:</strong> This directive overrides %URI.Base because
+    a given page may be on a sub-domain, but you wish HTML Purifier to be
+    more relaxed and allow some of the parent domains too.
+</p>
+--# vim: et sw=4 sts=4