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/URI/Email/SimpleCheck.php

@@ -0,0 +1,21 @@
+<?php
+
+/**
+ * Primitive email validation class based on the regexp found at
+ * http://www.regular-expressions.info/email.html
+ */
+class HTMLPurifier_AttrDef_URI_Email_SimpleCheck extends HTMLPurifier_AttrDef_URI_Email
+{
+
+    public function validate($string, $config, $context) {
+        // no support for named mailboxes i.e. "Bob <bob@example.com>"
+        // that needs more percent encoding to be done
+        if ($string == '') return false;
+        $string = trim($string);
+        $result = preg_match('/^[A-Z0-9._%-]+@[A-Z0-9.-]+\.[A-Z]{2,4}$/i', $string);
+        return $result ? $string : false;
+    }
+
+}
+
+// vim: et sw=4 sts=4