Replace existing autoloader with Composer-supplied

- Move Friendica namespace to `src`
- Move required `ezyang/htmlpurifier` to vendor
- Remove existing static autoloader
- Change boot.php reference to the autoloader

vendor/ezyang/htmlpurifier/library/HTMLPurifier.composer.php

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