Check manage_network when determining class selector in menu

Replaces use of is_super_admin to create a border between network
dashboard and the list of sites.
Only show network admin toggle when is_network_admin

wp-admin/network/ only
Use network_admin rather than super_admin in the POST data

The POST data that isn't being sent to the server right now :/
Wrap network admin text with label to make it clickable
If network_admin is not POSTed, revoke super admin privileges
Only show sites a user has access to in their menu