Add "Downgrade to previous version" selection

plugins/dynamix.plugin.manager/include/Downgrade.php

@@ -0,0 +1,18 @@
+<?PHP
+/* Copyright 2005-2018, Lime Technology
+ * Copyright 2012-2018, Bergware International.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License version 2,
+ * as published by the Free Software Foundation.
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ */
+?>
+<?
+$docroot = $docroot ?? $_SERVER['DOCUMENT_ROOT'] ?: '/usr/local/emhttp';
+exec("cp -rf /boot/previous/* /boot >/dev/null");
+exec("rm -rf /boot/previous >/dev/null");
+file_put_contents("$docroot/plugins/unRAIDServer/README.md","**DOWNGRADE TO VERSION {$_GET['version']}**");
+?>