USER STORY: Create the shipping address logic for the checkout page

[PedroMiguelFerreira]

As a user, I can access a shipping address form on the checkout page so that I can fill it out depending on whether my cart contains an item that needs to be shipped or not

[PedroMiguelFerreira]

• [x] Create a method for shipping on the Order model that checks if there are any items on the order that are not digital
• [x] Create the logic to define the order's shipping status by adding the shipping attribute to views.py
• [x] Create the logic to hide the shipping form or display it depending on what the shipping status/value is
• [x] Create an event handler that hides the form button from users, shows them the payment option and lets them edit the checkout form when they add data to it and click continue
• [x] Add demo payment button
• [x] Create an event handler and a function to trigger a payment action on submission