[api] Add display_price to products API

This allows for an easier rendering of the proper amount when using things like a JavaScript library to interact with the API
Fix backend order split shipments action

When moving an item to a existing shipment from the same stock location
it would make two requests to remove items from the same shipment.
Instead it needs to remove from one shipment and then add the same
quantity to another shipment

[Fixes #3161] [Fixes #3162]