Directly go from checkout to payment method when no shipping is required

When an Item requires no shipping method upon clicking checkout he will be directly redirected to payment method choice. 

      /checkout/ -> /checkout/payment-method/

Other items, which will require items will have 2 extra redirections as usual:

     /checkout/ -> /checkout/shipping-address/ -> /checkout/shipping-method/ -> /checkout/payment-method/

In order to test this, you can load into the sandbox website the fixtures in `kurse.json` which contain Course Items. This items require no shipping an can be checked out and paid as described above.