Iteration C2: Adding a Page Layout(application.html, application.css.scss)
Iteration C3: Using a Helper to Format the Price(number_to_currency)
Iteration C4: Functional Testing of Controllers
(using assert_select)
Iteration D1: Finding a Cart
(generate "cart" scaffold and modify application_controller
>+ def current_cart)
Iteration D2: Connecting Products to Carts
(has_many & belongs_to & before_destroy)
Iteration D3: Adding a Button
(button_to & line_items_controller.create
 & carts/show.html & test for line_items)