Clean up code by migrating ItemSoldInfo to Purchase model

Right now, the old `ItemSoldInfo` model (1 item was bought at the store) is used along with the new `Purchase/PurchasedItem` models (one purchase can have one or more items), and they are stored separately in the DB. This is redundant sale storage mechanisms. Everything should use the `Purchase/PurchasedItem` methodology, so use a database migration to move things over, and then get rid of all uses of `ItemSoldInfo`.