9.1 basic setup of a cart
LineItem as scaffold -> reference to product and belongs to cart
cart has many line items (set parent relationship)
product has many line_items; on destroy ensure that there are no line items
add button (add to cart) and use a set_cart on model for line item
line_item#create get the product from params and create a line item on cart for this product
modify the create line item test and correct error to let it pass
list cart items on cart#show
extrawork: playtime 9.4 done