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/Exception.php

@@ -0,0 +1,12 @@
+<?php
+
+/**
+ * Global exception class for HTML Purifier; any exceptions we throw
+ * are from here.
+ */
+class HTMLPurifier_Exception extends Exception
+{
+
+}
+
+// vim: et sw=4 sts=4