Stub product details and cart service

[robertlevy]

this adds a 'cart service' which centralizes keeping track of what's in the cart from clientside perspective.

used in the header to display a count of cart items

used on the product details page to enable the 'add to cart' button

also wired up the product details page to fetch data from services

[robertlevy]

todo:

• [ ] hook up calls to the 'add to cart' service
• [ ] navigate user to the cart page after they've added something to the cart
• [ ] make sure tests still run