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
Merge pull request #95 from bonobos/fix-line-item-availability-validator

Fix AvailabilityValidator for saved records