Only compute an amount no greater than the objects amount.
Compute amount based on remaining amount so that multiple promotion actions don't cause negative balance.
Fix AvailabilityValidator for saved records

Previously availability only considered inventory_units on the
target_shipment of a line_item. Since target_shipment is not persisted,
a saved line_item would become invalid when its variant ran out of
available inventory even if the inventory_units were already reserved.

Fixes #5208
This resolves #5132. The admin manual order cart is much more like the frontend now in that you start off by adding items to the cart(line items). You then add an address, the shipments are calculated based on the frontend rules you have set up but you as an admin can ignore those rules and muck with the shipments on the Shipments tab. You can also skip adding items to the cart altogether by just entering an address and then adding shipments manually, the way the old backend admin did before this patch.

Extracting the advanceOrder function into a function in the Spree namespace so that both the shipmentsJS and line_itemsJS can share it

Advancing the order after we add variants on the cart page. If we have already created shipments, then when we edit a line item the order gets reset to the address state. This call pushes it back to the payment state and destroys the old shipments, recreating them to match the new line item state.

In checkout.rb we first create_proposed_shipments but here we check if addresses are valid. If not we need to destroy_all instead of delete_all so that the inventory_units that were created by create_propsed_shipments will also be destroyed and not clutter up the DB

Fixing the Split tool. IF success we reload the page, else display a generic error. Would like to in the future provide more specific errors. Not allowing splits to the same shipment nor to a shipment or location that does not have the stock and is NOT backorderable. Next commit will have a boatload of automated tests.

Adding appropriate tests for the splitting of shipments.

Fixes #5166