Use the order label when saving a shipment instead of order number

A draft order that has a shipment created for it will result in a fatal error as there is no order number for the relevant placeholder in the save message presented to the admin on form submission. I don't know if / how this worked before, but it doesn't work now. The issue is that draft orders typically do not yet have order numbers assigned to them. We should switch to using the order label, which will correspond better to the page title, breadcrumbs, etc. anyways.