Fix line items taxing creation

Due to an association mismatch they were still being targed at the order
instead of line items as it should. tax_rate_spec was also checking
adjustments on orders instead of checking it on line_items
Simplify TaxRate `adjust` api

It receives a single item now as second param instead of a collection of
items
Include Order shipment_adjustments associations

Grab all order shipments adjustments in one query. That can help us
actually see on the UI which adjustments are created and at which
objects (order, line_item, shipment) in order summary as we move forward
on checkout
Update order totals in order `empty!`

Since we no longer have after_save callbacks calling order.update!