Fix order items data

[rossbowen]

Hey! Great project!

Running jafgen off the latest commit doesn't return order item data as expected - instead the data produced are copies of the different products.

I've reintroduced order items throughout with some slight changes to the codebase:

• Added OrderItem in order.py and changed the get_order_items return type for each of the customer personas.
• Order items now have a quantity field, indicating a customer can order multiples of a particular product.
• I've renamed Item to Product throughout to distinguish between a product-item and an order-item.