[Spree Upgrade] Fix subscriptions.shipping_method_id (13 broken specs)

[luisramos0]

Subscriptions and spree 2 shipping_methods will need to work together.
There's a column subscriptions.shipping_method_id that, I believe, is used to create orders in a subscription. We need to verify how this process works with the new spree 2 way of creating shipments from shipping methods.

Files referencing shipping_method_id:
- app/serializers/api/admin/subscription_serializer.rb (this used to be a separate issue #2732 that can be reopened if appropriate)
- app/models/proxy_order.rb
- app/services/order_syncer.rb
- app/assets/javascripts/admin/subscriptions/services/subscription_functions.js.coffee
- app/views/admin/subscriptions/_details.html.haml
- app/views/admin/subscriptions/_table.html.haml
- spec/controllers/admin/subscriptions_controller_spec.rb
- spec/services/subscription_form_spec.rb
- spec/services/order_syncer_spec.rb
- spec/features/admin/subscriptions_spec.rb
- spec/controllers/admin/proxy_orders_controller_spec.rb
- spec/javascripts/unit/admin/subscriptions/services/subscription_functions_spec.js.coffee

[luisramos0]

Moving app/services/order_factory.rb and respective spec/services/order_factory_spec.rb from #2693 to this issue as it's related to subscriptions only.
I'll fix all other parts referencing shipping_method before getting to subscriptions.

[luisramos0]

After a lot more handling of shipping_methods and shipments and shipping_method_ids, I suspect this shipping_method_id in subscriptions is simply holding the shipping_method_id to be used in the recurring orders, and that can be done by injecting the shipping_method_id in the order.shipment, just like it's done in the checkout controller in #2683
Based on this assumption, I am leaving this verification/adaptation for later.

[luisramos0]

One spec in subscription_form_spec is broken with errors related to shipments.
And 8 specs are failing with related errors in spec/services/order_syncer_spec.rb

[luisramos0]

Additionally, 4 specs are breaking with similar error (we need to set shipping address or shipping method at some point) in spec/models/proxy_order_spec (job 4)

Error:

ensure_available_shipping_rates shipping_method…
Cannot transition state via :next from :address (Reason(s): We are unable to ship the selected items to your shipping address. Please choose another shipping address.)