[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
Here the code need cleaning. You are mixing
- computation of expired coupon
- check of coupon state
- 'no coupon' error that I don't clearly understand

You should
1/ check for state
2/ apply
3/ post-process if necessary, probably in the controller