[IMP] sale_coupon: adapt to multi-websites
[IMP] sale_coupon: adapt to multi-websites

This commit adds the `website.multi.mixin` to coupon program to make it
multi-websites.

It also adds the website_id field in the views and adapt SQL constraint on code
which now became multi-websites friendly.

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo