Free Orders should skip payment step

[stephenmcd]

Eg, cart contains items that are $0, and shipping is free - in this case the payment step should be skipped.

Will involve tweaks to the structures that control the checkout steps, since one will be removed dynamically, which doesn't happen anywhere yet.

[Remiz]

Do you think this should come as an option like SHOP_DISABLE_PAYMENT_IF_ZERO or should it be skipped by default?

I noticed that if I pass an order with $0 using the Stripe payment handler, Stripe API returns an error Invalid positive integer, so I'm wondering if it should be enabled by default...

[stephenmcd]

Do you think this should come as an option like SHOP_DISABLE_PAYMENT_IF_ZERO

no

[Remiz]

Here is my attempt at the problem
#307