Allow to define a custom shipping date on sale

[ced (Cédric Krier)]

@dave

This may not be relevant, but there are sectors I know of where it is common for customers to place orders, and on the order they will specify the date, or week, that the goods must be delivered.

Indeed I forgot to talk about this case. For me, it is about registering a “not before” date on the order. So it is another constraint to take when planning shipments.