strip_tags_long: use htmlpurifier to properly reformat html content

lib/htmlpurifier/library/HTMLPurifier/ConfigSchema/schema/HTML.AllowedElements.txt

@@ -0,0 +1,18 @@
+HTML.AllowedElements
+TYPE: lookup/null
+VERSION: 1.3.0
+DEFAULT: NULL
+--DESCRIPTION--
+<p>
+    If HTML Purifier's tag set is unsatisfactory for your needs, you
+    can overload it with your own list of tags to allow.  Note that this
+    method is subtractive: it does its job by taking away from HTML Purifier
+    usual feature set, so you cannot add a tag that HTML Purifier never
+    supported in the first place (like embed, form or head).  If you
+    change this, you probably also want to change %HTML.AllowedAttributes.
+</p>
+<p>
+    <strong>Warning:</strong> If another directive conflicts with the
+    elements here, <em>that</em> directive will win and override.
+</p>
+--# vim: et sw=4 sts=4