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/HTMLModule/Tidy/Strict.php

@@ -0,0 +1,43 @@
+<?php
+
+class HTMLPurifier_HTMLModule_Tidy_Strict extends HTMLPurifier_HTMLModule_Tidy_XHTMLAndHTML4
+{
+    /**
+     * @type string
+     */
+    public $name = 'Tidy_Strict';
+
+    /**
+     * @type string
+     */
+    public $defaultLevel = 'light';
+
+    /**
+     * @return array
+     */
+    public function makeFixes()
+    {
+        $r = parent::makeFixes();
+        $r['blockquote#content_model_type'] = 'strictblockquote';
+        return $r;
+    }
+
+    /**
+     * @type bool
+     */
+    public $defines_child_def = true;
+
+    /**
+     * @param HTMLPurifier_ElementDef $def
+     * @return HTMLPurifier_ChildDef_StrictBlockquote
+     */
+    public function getChildDef($def)
+    {
+        if ($def->content_model_type != 'strictblockquote') {
+            return parent::getChildDef($def);
+        }
+        return new HTMLPurifier_ChildDef_StrictBlockquote($def->content_model);
+    }
+}
+
+// vim: et sw=4 sts=4