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/plugins/phorum/migrate.bbcode.php

@@ -0,0 +1,31 @@
+<?php
+
+/**
+ * This file is responsible for migrating from a specific markup language
+ * like BBCode or Markdown to HTML. WARNING: THIS PROCESS IS NOT REVERSIBLE
+ *
+ * Copy this file to 'migrate.php' and it will automatically work for
+ * BBCode; you may need to tweak this a little to get it to work for other
+ * languages (usually, just replace the include name and the function name).
+ *
+ * If you do NOT want to have any migration performed (for instance, you
+ * are installing the module on a new forum with no posts), simply remove
+ * phorum_htmlpurifier_migrate() function. You still need migrate.php
+ * present, otherwise the module won't work. This ensures that the user
+ * explicitly says, "No, I do not need to migrate."
+ */
+
+if(!defined("PHORUM")) exit;
+
+require_once(dirname(__FILE__) . "/../bbcode/bbcode.php");
+
+/**
+ * 'format' hook style function that will be called to convert
+ * legacy markup into HTML.
+ */
+function phorum_htmlpurifier_migrate($data)
+{
+    return phorum_mod_bbcode_format($data); // bbcode's 'format' hook
+}
+
+// vim: et sw=4 sts=4