Rename Carts to CartController.
Document controllers.CartController.add.
Merge branch 'extend-shop' into cart-controller-refactoring
Add missing import.
Make controllers.CartController.add an instance method.
Refactor controllers.CartController.add to use only one return statement.
Refactor controllers.CartController.add, use services.
Refactor CartController.
Refactor controllers.CartController.update, use if/else and promises.
Refactor controllers.CartController.update to use services.
Refactor controllers.CartController.{remove,show} to use services.
Remove tests since they will replaced with better ones.