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/Language/classes/en-x-test.php

@@ -0,0 +1,9 @@
+<?php
+
+// private class for unit testing
+
+class HTMLPurifier_Language_en_x_test extends HTMLPurifier_Language
+{
+}
+
+// vim: et sw=4 sts=4