[frontend] Save an order in OrdersController#update after update_cart is called
[frontend] Work with in-memory objects for eligible shipment adjustments in checkout/_summary
[frontend] Correct definition for eligible_promotion_adjustments in _order_details
[api] Save an order after applying a coupon code

The Handler will create a non-persisted Adjustment record, and saving the record will save the adjustment and persist the totals.
Only save the order once in OrderContents#update
[backend] Reference in-memory adjustments in Admin::AdjustmentsController
[backend] Reference in-memory adjustments in orders/_adjustments.html.erb