init idea on using ecto for model layer

[btkostner]

Right now our data gets pretty messy after the API layer. This uses ecto to improve casting data into structs, and replaces all of the different session data with a single Elementary.Store.Checkout.Cart struct.

• This includes an auto generated ID to replace session_id
• Should include functions to convert stripe checkout session to a cart
• Should include functions to convert a printful order to a cart
• We should use the struct to pass all of the cart information around
• We should do more data validation in ecto and handle changeset errors better