Implement a state machine in Fulfillment

We already have a state machine in Order, Payments and Refunds, I believe that we should implement that in the Fulfillment too so that we can make things like: create labels for shipping, create tracking code (this depends on carrier), notify users of the state of the fulfillment by email and many other things…

Features

• Implement a state machine in Fulfillment. I thought in these states initially: 'Pending' | 'Shipped' | 'Delivered' | 'Cancelled'
• Button in Admin UI to change the Fulfillment state (like Payment and Refunds)
• A way to define how the Fulfillment should work when transiting between states

[michaelbromley]

Status update

The PR implementing the server-side of this is now merged into next.

Tasks outstanding:

• Admin UI parts to interact with Fulfillment state
• Documentation

[jonyw4]

@michaelbromley I little update here, when I started to update the order e2e tests I realize that we will need PartiallyShipped order state too. Today If you forget to fulfill an order item, the order state will continue in PartiallyFulfilled. We need to achieve that in this PR too. So the new order states must be that:

PartiallyShipped

To reach this state, the Order must have at least one Fulfillment in Shipped state.

Shipped

To reach this state all order items must have a Fulfillment and all Fulfillment must be in Shipped state

PartiallyDelivered (old PartiallyFulfilled)

To reach this state, the Order must have at least one Fulfillment in Delivered state.

Delivered (old Fulfilled)

To reach this state all order items must have a Fulfillment and all Fulfillment must be in Delivered state