Add created/posted At date to products

It would be nice to see when a product was first set to public by the creator, separate from the Product Contents. Adding this would help people know when something was first posted and also see whether the creator has updated it since the initial release. Ideally, if the product is later set to unlisted or private, this should not change the Created/Posted At date; the date should only be recorded the first time the product is made public.