Store voucher codes and PDF url in snapshot

[TomDeSmet]

What are you trying to do?
On my order screen I show the giftcards that a customer bought. But if the giftvoucher has been deleted from the system (say due to a lack of interest) the codes and PDF cannot be obtained anymore as these methods only work via the purchasable which does not exist on a deleted element.

So if the user hasn't used his code yet and didn't print the code out, he can never obtain his voucher anymore.