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/FrameTarget.php

@@ -0,0 +1,21 @@
+<?php
+
+/**
+ * Special-case enum attribute definition that lazy loads allowed frame targets
+ */
+class HTMLPurifier_AttrDef_HTML_FrameTarget extends HTMLPurifier_AttrDef_Enum
+{
+
+    public $valid_values = false; // uninitialized value
+    protected $case_sensitive = false;
+
+    public function __construct() {}
+
+    public function validate($string, $config, $context) {
+        if ($this->valid_values === false) $this->valid_values = $config->get('Attr.AllowedFrameTargets');
+        return parent::validate($string, $config, $context);
+    }
+
+}
+
+// vim: et sw=4 sts=4