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/AttrTransform/Lang.php

@@ -0,0 +1,31 @@
+<?php
+
+/**
+ * Post-transform that copies lang's value to xml:lang (and vice-versa)
+ * @note Theoretically speaking, this could be a pre-transform, but putting
+ *       post is more efficient.
+ */
+class HTMLPurifier_AttrTransform_Lang extends HTMLPurifier_AttrTransform
+{
+
+    /**
+     * @param array $attr
+     * @param HTMLPurifier_Config $config
+     * @param HTMLPurifier_Context $context
+     * @return array
+     */
+    public function transform($attr, $config, $context)
+    {
+        $lang = isset($attr['lang']) ? $attr['lang'] : false;
+        $xml_lang = isset($attr['xml:lang']) ? $attr['xml:lang'] : false;
+
+        if ($lang !== false && $xml_lang === false) {
+            $attr['xml:lang'] = $lang;
+        } elseif ($xml_lang !== false) {
+            $attr['lang'] = $xml_lang;
+        }
+        return $attr;
+    }
+}
+
+// vim: et sw=4 sts=4