[ADD] CDN support for website
[FIX] product: remove empty from state field selection

In order to avoid having twice "empty" in the web client. There is always an empty option in selection field in the web client. When the selection field is required, you have to select it, and it is not required, you can set False as selection value
[ADD] website_sale_coupon : allows to give a coupon to customer, with validity, available for some products