7.1 Use window local storage for saving cart data (for both guests and logged in user)

[alliebug]

Essential for #32

• • For guests (with no account), cart items will persists on that particular device (because of local storage)
    • if guest tries to view website/cart on a diff device, they would not see their cart items from before
• For Users with account: save the cart info (from local storage) to the database
  • when user with account logs out on the current device,
    • the cart should no longer display info.
    • (and then if user log in on another device, they will still see their cart items)
  • if user adds stuff to cart, and THEN log in, the cart items should be combined with any existing items from before