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/extras/HTMLPurifierExtras.php

@@ -0,0 +1,31 @@
+<?php
+
+/**
+ * Meta-class for HTML Purifier's extra class hierarchies, similar to
+ * HTMLPurifier_Bootstrap.
+ */
+class HTMLPurifierExtras
+{
+
+    public static function autoload($class)
+    {
+        $path = HTMLPurifierExtras::getPath($class);
+        if (!$path) return false;
+        require $path;
+        return true;
+    }
+
+    public static function getPath($class)
+    {
+        if (
+            strncmp('FSTools', $class, 7) !== 0 &&
+            strncmp('ConfigDoc', $class, 9) !== 0
+        ) return false;
+        // Custom implementations can go here
+        // Standard implementation:
+        return str_replace('_', '/', $class) . '.php';
+    }
+
+}
+
+// vim: et sw=4 sts=4