add cart clear method
defined remove cart item function

- rails g controller cart_items
- def destroy function
- route.rb 加入雙層 resource :items 並指定 controller 為 cart_items
- 於 cart/index.erb 加入刪除功能, 使用 bootstrap icon and change cart.items (product) to cart.cart_items
update gitignore
remove .idea
cart_item add quantity

- rails g migration add_quantity_to_cart_items
    add_column :cart_items, :quantity, :integer, default: 1
- cart.rb adjust add_product_to_cart function
- products_controller adjust add_to_cart
update total price

change use cart_items.product.price * cart_items.quantity
cart_items can choose quantity

- cart_item add def update
- cart/index.html.erb add select quantitiy
  use form_for / f:select / s.submit
fix cache item qunatity
add product amount check in product detail

- check product.quantity in def add_to_cart
- in products/show.erb.html , check the product quantity