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

library/ezyang/htmlpurifier/library/HTMLPurifier/ConfigSchema/Exception.php

@@ -1,11 +0,0 @@
-<?php
-
-/**
- * Exceptions related to configuration schema
- */
-class HTMLPurifier_ConfigSchema_Exception extends HTMLPurifier_Exception
-{
-
-}
-
-// vim: et sw=4 sts=4