Add the app_id

This is necessary for the javascript SDK

Author: jonasdt

config/mailium-oauth.php

@@ -9,6 +9,7 @@
     | App credentails
     */
 
+    'app_id' => env('MAILIUM_APP_ID', ''),
     'client_id' => env('MAILIUM_APP_CLIENT_ID', ''),
     'client_secret' => env('MAILIUM_APP_CLIENT_SECRET', ''),