Escape story fields and clean story HTML with HTML Purifier.

Add HTML Purifier 4.19.1. Stories are purified when they are saved and
when they are shown.

lib/htmlpurifier-4.19.1/library/HTMLPurifier/ConfigSchema/schema/HTML.ForbiddenAttributes.txt

@@ -0,0 +1,21 @@
+HTML.ForbiddenAttributes
+TYPE: lookup
+VERSION: 3.1.0
+DEFAULT: array()
+--DESCRIPTION--
+<p>
+    While this directive is similar to %HTML.AllowedAttributes, for
+    forwards-compatibility with XML, this attribute has a different syntax. Instead of
+    <code>tag.attr</code>, use <code>tag@attr</code>. To disallow <code>href</code>
+    attributes in <code>a</code> tags, set this directive to
+    <code>a@href</code>. You can also disallow an attribute globally with
+    <code>attr</code> or <code>*@attr</code> (either syntax is fine; the latter
+    is provided for consistency with %HTML.AllowedAttributes).
+</p>
+<p>
+    <strong>Warning:</strong> This directive complements %HTML.ForbiddenElements,
+    accordingly, check
+    out that directive for a discussion of why you
+    should think twice before using this directive.
+</p>
+--# vim: et sw=4 sts=4