product add price
Add product to cart_item

generate model
* cart
* cart_item

current_cart
carts#index
Build order, order_info, order_item
Change style
Tidy up product price
orders#show
simplify conditional
Push user is_admin
#pay_with_credit_card
order include AASM
#make_payment!
[ci skip] some comments
Use #find_or_create_by in here
current_cart.clear!
delete single cart item
cart_item add_column :quantity
total_price = price * quantity
reject buy quantity equal zero at product
order_item.quantity = cart_item.quantity