[Spree 2 Upgrade] Refactor Order#shipping_method in checkout flow

[sauloperez]

Description

The customization we added in the payment step of the checkout flow relies on Order#shipping_method which is gone in Spree 2.0. Therefore, this logic needs to be refactored to work on that Spree version.

openfoodnetwork/app/models/spree/order_decorator.rb

Lines 33 to 42 in 09534b4

	go_to_state :payment, :if => lambda { |order|
	# Fix for #2191
	if order.shipping_method.andand.delivery?
	if order.ship_address.andand.valid?
	order.create_shipment!
	order.update_totals
	end
	end
	order.payment_required?
	}

[sauloperez]

We need to check the status of this issue now that we got #2431

[luisramos0]

I am going to close this issue as the main problem in its description (the checkout workflow) is superbly fixed with the beautiful (and simple!) #2431
I have created #2683 which will handle the rest of the problem. I'll describe my understanding of what needs to be done in #2683