Disallow managing roles with greater privileges

www.agileplannerapp.com/boards/173808/cards/5254

* Only allow superadmins to view and manage other superadmins
* 'Filter by Role' only shows roles that current user can manage
I don't this this is correct any more - Admins should be able to see other Admins, just not SuperAdmins.  I guess it's spuriously passing because of the issue I mentioned in the `role_not_present` method below.