Apply the case-insensitive network-site comparisons added in [27725] for `wp-admin/network/admin.php` to `wp-admin/user/admin.php` too, for consistency.
Props rimadoshi.
Fixes#49300.
git-svn-id: https://develop.svn.wordpress.org/trunk@47115 602fd350-edb4-49c9-b593-d223f7449a82