Filter links on network/users.php for All | Super Admins

[nacin]

In MU, setting up super admins was an ugly input box with logins separated by spaces. In 3.0, this was much improved with a checkbox on user-edit and abstraction to grant|revoke functions.

Unfortunately, the changes in 3.0 make it impossible to list every user who is a super admin in one place, unless you're using the $super_admins define. It's kind of a security issue.

I'd like to add filter links to network/users.php that allow filtering down to the super admins.

[nacin]

(In [16277]) Super Admin filter for network/users.php. fixes #15371.