improve override of /api/orders/show

this replaces https://github.com/spree/spree_hub_connector/pull/20.

spree_hub_connector was dropping some things that spree had included, like [display_item_total](https://github.com/spree/spree/blob/b0d5242/api/app/views/spree/api/orders/order.v1.rabl#L2)

we used to:

- override the global "spree/api/orders/show" view
- override the global "order_attributes" helper method

and now instead we:

- add an integrator "spree/api/integrator/show_order" view
- override the integrator "order_attributes" helper method