added kopano & postifx configuration files

etc/kopano/webapp/config-mattermost.php

@@ -0,0 +1,9 @@
+<?php
+
+define('PLUGIN_MATTERMOST_USER_DEFAULT_ENABLE', false);
+
+define('PLUGIN_MATTERMOST_URL', '<URL-OF-YOUR-MATTERMOST>');
+
+// This setting can be changed by the user in his settings.
+// Here you can define the default behaviour.
+define('PLUGIN_MATTERMOST_AUTOSTART', true);