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/extras/HTMLPurifierExtras.auto.php

@@ -0,0 +1,11 @@
+<?php
+
+/**
+ * This is a stub include that automatically configures the include path.
+ */
+
+set_include_path(dirname(__FILE__) . PATH_SEPARATOR . get_include_path() );
+require_once 'HTMLPurifierExtras.php';
+require_once 'HTMLPurifierExtras.autoload.php';
+
+// vim: et sw=4 sts=4