[MW] Completed Cart feature

[mayawang]

Completed Cart feature logic.

As a guest, I can:

• Add in-stock products to my cart
• Select the quantity of product to add to my cart
• The quantity to add to my cart is a dropdown list from 1 up to product inventory
• Click order item name to see details of each order item in my cart
• Change the quantity of an existing product in my cart
• The same products added in cart in separate occasions will be merged into a single order item
• Remove each product from my cart
• Show subtotal price of all order items in my cart
• Button to "continue shopping", will direct to homepage
• Button to "checkout", will direct to checkout page(the start of purchase feature, currently under construction)

Major change:

• changed Order model status attribute from boolean to string (pending and paid)