[IMP] website_sale, website_sale_event: adapt views to respetc the show_discount

Now, the user can choose on the pricelist if the price should be displayed as a
discount or not (price striked red).

cart_update and confirm_order recompute unit price by default.
So we need for website_event_sale to be able to ask to don't recompute the price.
Else the price of product will be used instead of the price of ticket.