Shop manager challenge

[snptrs]

This is functioning and satisfies the user stories. The tests are all passing.

With a bit more time, next steps would be:

• Implement some error checking. None of the input is being tested at the moment, so it will error out if an unexpected value is entered.
• Allow multiple units of an item to be added to an order. Something to do with how I've set up the items_orders join table is only allowing it to take one unique item for each order (I think it's because the order id and item id are both primary keys in there).
• Not entirely convinced that all my IO tests are testing what I think they're testing. Need to do some research into how to better test IO when there are lots of inputs and outputs in a method.