Add an expected delivery date for shipments

I think we should start with a shipped timestamp which is set on send transition.

The changed timestamp should be kept distinct from the shipped timestamp, as you may make edits to the shipment before you send it.

I also think it would be good to add a delivered timestamp which is set on a new deliver transition.

Then websites that use carrier webhooks or paying for updates can set delivered timestamps accordingly in their own code.

I think the original issue would better be handled by trying to support the major carriers in callbacks to get those times dynamically.