Enhancement: Add scanning per user

[Deadpikle]

We should be able to somehow add customers to the database and scan their items in such a way that we can look up a customer's order/purchase history.

[Deadpikle]

This is now mostly done. Customer data needs to be moved to its own separate table (currently in Purchases table), we need to migrate ItemSoldInfo to the Purchases table (see #9), and we should be able to run reports on specific customers.