Clean up code by migrating ItemSoldInfo to Purchase model

[Deadpikle]

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.