Add website to client repository

Closes #1567

The main reason I'm not simply merging Annika's PR is because this
way makes it clearer that I'm taking responsibility for all this
code, that it's mostly code I wrote, and also because it makes it
easier to ensure that none of the files have been changed.

(Not that I don't personally trust Annika, but I have something
resembling an obligation to users not to expose them to risks based
on personal trust.)

website/lib/htmlpurifier/HTMLPurifier/AttrDef/HTML/Class.php

@@ -0,0 +1,34 @@
+<?php
+
+/**
+ * Implements special behavior for class attribute (normally NMTOKENS)
+ */
+class HTMLPurifier_AttrDef_HTML_Class extends HTMLPurifier_AttrDef_HTML_Nmtokens
+{
+    protected function split($string, $config, $context) {
+        // really, this twiddle should be lazy loaded
+        $name = $config->getDefinition('HTML')->doctype->name;
+        if ($name == "XHTML 1.1" || $name == "XHTML 2.0") {
+            return parent::split($string, $config, $context);
+        } else {
+            return preg_split('/\s+/', $string);
+        }
+    }
+    protected function filter($tokens, $config, $context) {
+        $allowed = $config->get('Attr.AllowedClasses');
+        $forbidden = $config->get('Attr.ForbiddenClasses');
+        $ret = array();
+        foreach ($tokens as $token) {
+            if (
+                ($allowed === null || isset($allowed[$token])) &&
+                !isset($forbidden[$token]) &&
+                // We need this O(n) check because of PHP's array
+                // implementation that casts -0 to 0.
+                !in_array($token, $ret, true)
+            ) {
+                $ret[] = $token;
+            }
+        }
+        return $ret;
+    }
+}