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/Token/Start.php

@@ -0,0 +1,10 @@
+<?php
+
+/**
+ * Concrete start token class.
+ */
+class HTMLPurifier_Token_Start extends HTMLPurifier_Token_Tag
+{
+}
+
+// vim: et sw=4 sts=4