[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
Several issues here. You are in a multi environment, and checking self.id . Moreover, if working on a single sale_order, you have access to its order_line that are its order lines. So either recomputed as a true multi, or add an ensure_one.