Fix casing consistency of $TAB on login page (#2883)

This was made lowercase in #926, not sure why.

web/login/index.php

@@ -5,8 +5,7 @@ define('NO_AUTH_REQUIRED', true);
 
 include($_SERVER['DOCUMENT_ROOT'] . '/inc/main.php');
 
-$TAB = 'login';
-
+$TAB = 'LOGIN';
 
 if (isset($_GET['logout'])) {
     unset($_SESSION);