Introduce a get_global_admins method

Retrieves a list of current global admins, should have a more
public facing function at some point.
Introduce is_global_admin method

Determine if a user is a global admin
Make use of the new get_global_admins when setting super admins
s/super_admins/network_admins/ - replaced incorrect array
Display an option to assign super admin privs to a user.

This is only shown to global admins and replaces the previous
functionality for standard super admins.