Create cart & cartlineitem table to contain the cart information

[chrisK00]

Suggested design from the discord thread:

public class Cart
{
    public int Id { get; set; }
    public ICollection<CartItem> CartItems { get; set; }
}

public class CartItem
{
    public int Id { get; set; }
    public int CartId { get; set; }
    public Product Product { get; set; }
    public int Quantity { get; set; }
}

child of #70