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.composer.php

@@ -0,0 +1,4 @@
+<?php
+if (!defined('HTMLPURIFIER_PREFIX')) {
+    define('HTMLPURIFIER_PREFIX', __DIR__);
+}