Reorganize PHP internals and static assets

Move shared PHP code into private/, move JavaScript files into js/, and block direct access to private/. Remove unused API key and cache artifacts from the working tree.

private/config/base64-chunks.php

@@ -0,0 +1,12 @@
+<?php
+/*
+ * Maximale grootte van base64-payloads, in KiB.
+ *
+ * 1 KiB = 1024 bytes. De splitter rondt intern waar nodig naar beneden af
+ * zodat de binaire chunks netjes naar base64 omgezet kunnen worden.
+ */
+
+return array (
+  'racket_zip_max_base64_kb' => 10240,
+  'package_zip_max_base64_kb' => 2048,
+);