Admin users should be able to set any param via api importer
Add items before creating shipments on importer

Otherwise order.contents.add would wipe out shipments every time
Try to set inventory_unit.line_item on Importer

To avoid exception when users want to display those imported orders in
admin
Fix shipment persistence on order importer

Besides selecting the shipment rate we also need to assign the cost in
the shipment itself.

We cant only rely on order.update_shipment_state to actually well update
the order shipment_state (:O) because shipment states might not be sync
with the latest payment changes which influence on the result so
Shipment#determine_state needs to run to make sure we keep those
accurate