Add message if user is kicked out of application for lack of permissions

index.php

@@ -1,5 +1,4 @@
 <?php
-
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
@@ -9,10 +8,14 @@ require_once __DIR__ . "/required.php";
 require_once __DIR__ . "/lib/login.php";
 
 // if we're logged in, we don't need to be here.
-if ($_SESSION['loggedin']) {
+if ($_SESSION['loggedin'] && !isset($_GET['permissionerror'])) {
     header('Location: app.php');
 }
 
+if (isset($_GET['permissionerror'])) {
+    $alert = lang("no permission", false);
+}
+
 /* Authenticate user */
 $userpass_ok = false;
 $multiauth = false;

lang/en_us.php

@@ -28,6 +28,7 @@ define("STRINGS", [
     "login server error" => "The login server returned an error: {arg}",
     "login server user data error" => "The login server refused to provide account information.  Try again or contact technical support.",
     "captcha error" => "There was a problem with the CAPTCHA (robot test).  Try again.",
+    "no permission" => "You don't have permission to do that.",
     "home" => "Home",
     "more" => "More",
     "test" => "Test"