Ask for check digits on negative stock levels

[sde1000]

At the moment, when you sell stock and this leads to the amount in stock becoming negative, the register pops up a red warning box prompting you to check with your manager and press "Use Stock" to continue.

This warning box is routinely ignored at some sites.

There's not much we can do for continuous stock lines, but for regular stock lines we could instead:

• Don't process the sale until the box is dismissed (rather than have the sale appear under the box as happens at present)
• Require the check digits from the label on the item being sold to be entered to dismiss the box (rather than just "Use Stock" as at present)
• Have "Use Stock" go to the "Use Stock" UI for the stock line, i.e. prompt the user what to do with the finished stock and then ask for the stock label details from the new stock item

We would need to make this configurable: firstly whether it is turned on at all, and secondly the negative amount of stock remaining at which it kicks in.