Make Order#price_adjustments not iterate over each line item and load its adjustments one by one
Delete all inventory units (rather than destroy)

When a shipment was previously destroyed, it would call destroy on each InventoryUnit object. This took a long time if there were a lot of inventory units. There's no need to call destroy, as there's no callbacks in this model for destroy. All shipment needs to do is *DELETE ALL* its inventory units, and so this is what's going to happen now.
Only call update_order + update_inventory callbacks for LineItem if is destroyed or changed
Delete all shipping adjustments immediately prior to deleting all shipments in Order#create_proposed_shipments

What would happen before is that each shipment would find its adjustments and call destroy on them. We don't necessarily care about all that happening because once the new adjustments have been created then the order will be updated correctly.
Fixed bad style escaping within admin/products/index.html.erb

Fixes #3439