[backend] Improve display of usage information for promotions
Allow adjustments to be modified by Adjustment#update, unless they are closed?

This commit removes the confusing "finalized" state. The code was not using the "closed" state anywhere, but was using "finalized". It's clearer switching between "open" and "closed" than it is between "open" and "finalized", so the code now uses "open" and "close".
Added FreeShipping Promotion Handler

Whenever an order transitions from the delivery step, this handler will run and attempt to apply any free shipping promotions