[ADD] website_sale_coupon : allows to give a coupon to customer, with validity, available for some products

- Task: https://www.odoo.com/web#id=9507&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.ShuFMqJRjlBKDtpo
Move this code in a separate sale_order.py file. This way we know that the sale order modifications are held in its own file.