Other users in a network can now be given capabilities to manage users without also having global super admin privileges.
* Users with `manage_network_users` can not edit super admins.
* Users with `manage_network_users` can not promote users to super admin.
* Uses of `is_super_admin()` in `user-new.php` are now updated to `manage_network_users`.
Props daniellandau, chriscct7.
Fixes#16860.
git-svn-id: https://develop.svn.wordpress.org/trunk@33988 602fd350-edb4-49c9-b593-d223f7449a82