Allow user to cancel latest bought item

[MathiasMehl]

This could be useful when a user pays for an item, and it turns out the item is not in stock, or simply that a user wants another item instead.

The stregsystem already trusts and assumes that the user will not cheat, so this feature should not encourage cheating the stregsystem.

There could be a fixed time that a cancel is possible, e.g 1 min after buying an item.

[hrjakobsen]

If this is implemented, we have to figure out how to manage the accounting for this. This would probably entail creating a sale with a negative price, as well as adding a payment to the user, as to retain the correctness of the logged sales information.

[jones-g]

I would not say I'm completely against this but usually this has been "fixed" by simply picking up the purchased product when it is in stock again.