Refactor using RESTX and Swagger

[EricYoung37]

• The app is refactored from REST to RESTX.

• It now supports Swagger API documentations on localhost:8000/apidocs. Link also avialable at the bottom of the root localhost:8000.

• Restock action no longer takes user-input number to add to item quantity (user-input number can be assigned to item quantity via update method). The restock logic is resolved internally in the backend when the restock button is clicked. The idea is: the more we sold, the more we add, vice versa. More sophisticated algorithm may be adopted in the future.

if item.quantity < item.restock_level:
    item.quantity = 2*item.restock_level - item.quantity  # = item.restock_level + (item.restock_level - quantity)

• Code achieves 95% PyTest coverage.

• Code passed all behave tests.

• Added BDD badge in README

• Updated README with documentations for restcok action

• Updated README with documentations for two model attributes condition and restock_level