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.
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