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.autoload.php

@@ -0,0 +1,27 @@
+<?php
+
+/**
+ * @file
+ * Convenience file that registers autoload handler for HTML Purifier.
+ * It also does some sanity checks.
+ */
+
+if (function_exists('spl_autoload_register') && function_exists('spl_autoload_unregister')) {
+    // We need unregister for our pre-registering functionality
+    HTMLPurifier_Bootstrap::registerAutoload();
+    if (function_exists('__autoload')) {
+        // Be polite and ensure that userland autoload gets retained
+        spl_autoload_register('__autoload');
+    }
+} elseif (!function_exists('__autoload')) {
+    function __autoload($class)
+    {
+        return HTMLPurifier_Bootstrap::autoload($class);
+    }
+}
+
+if (ini_get('zend.ze1_compatibility_mode')) {
+    trigger_error("HTML Purifier is not compatible with zend.ze1_compatibility_mode; please turn it off", E_USER_ERROR);
+}
+
+// vim: et sw=4 sts=4