Adds initial super admin support
Adds suport for toggling super admin through backend

* Also corrects the location of the `normal_admin_users_spec` and the
`super_admin_users_spec`. These were living in `spec/api`, but they
were using Capybara, so they've been moved to `spec/features`.
Adds additional security for super admins
Wraps up super admin support

* The first admin created is a super admin
* Regular admin users are not show edit or delete links for super
  admin users
Adds support for a super admin that can manage all tenants

* The first user that's created will be a super admin
* Super admin users are able to create new users as super admin
  and to promote other admin users to super admin
* Each super admin user can log into every tenant
* Normal admins are tenanted just like non-admin accounts