F/cart backend for real

[brynn]

• [x] getCart instance method on User model
• [x] /api/users/:userId/cart api routes (GET and PUT)
• [x] working setCart() thunk and addToCart() thunk
• [x] util function for counting total cart items
• [x] dispatches setCart() from top-level component
• [x] creates new row in Orders table (representing an empty cart) when user logs in for the first time
• [x] LOGGED IN USER CART PERSISTS NO MATTER WHAT !! REFRESH THE PAGE AS MUCH AS YOU WANT AND REVEL IN THE MIRACULOUS PERSISTENCE OF THE CART (thanks @geoffbass!)
• [ ] no tests yet SORRY, tomorrow!
• [ ] local storage for non logged in users COMING SOON!