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/URIScheme/https.php

@@ -0,0 +1,18 @@
+<?php
+
+/**
+ * Validates https (Secure HTTP) according to http scheme.
+ */
+class HTMLPurifier_URIScheme_https extends HTMLPurifier_URIScheme_http
+{
+    /**
+     * @type int
+     */
+    public $default_port = 443;
+    /**
+     * @type bool
+     */
+    public $secure = true;
+}
+
+// vim: et sw=4 sts=4