Feature/order processes

[nwarsalee]

Added all the required backend processes for after an order is processed.

• Books in the order get associated to the order by its order number
• Book stock level of each book in the order is decreased by 1
• Publisher bank account for each book has its amount increased by the price of the book and the % to publisher

As a result, new queries and service functions have been created and the DB model has been altered to help these changes.

Closes #47 and closes #41