Add hooks for network and global capabilities when editing users

This will aid in getting current users assigned proper capabilities
before things get too out of hand.
Manage super admins through the $super_admins global

This disables the default core functionality for determining who
is a super admin and provides a custom array of users instead.

By doing this, we grab more granual control over the process. We
also lose the ability to assign super admins on the fly. This is
a good thing, as we can start to create that experience around
the concept of a global admin.