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/Empty.php

@@ -0,0 +1,15 @@
+<?php
+
+/**
+ * Concrete empty token class.
+ */
+class HTMLPurifier_Token_Empty extends HTMLPurifier_Token_Tag
+{
+    public function toNode() {
+        $n = parent::toNode();
+        $n->empty = true;
+        return $n;
+    }
+}
+
+// vim: et sw=4 sts=4