[FIX] website_sale: discount

- Go to Sales > Configuration > Settings
- Enable "Multiple Sales Prices per Product" and select option "Prices
  computed from formulas (discounts, margins, roundings)"
- Go to Sales > Catalog / Pricelists
- Select "Public Pricelist" and set "Discount Policy: Show public price
  & discount to the customer"
- Go to the eCommerce, buy "iPad Retina Display" which as a 20 %
  discount

In the corresponding SO, the discount appears to be zero, and the
discount is included in the price unit. However, adding the product
directly to the SO in the backend displays the discount as expected.

There should not be a difference in behavior between the eCommerce and
the backend.

Fixes #31337
opw-1943989

closes odoo/odoo#31965

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/website_sale/models/sale_order.py

@@ -98,11 +98,26 @@ class SaleOrder(models.Model):
             'pricelist': order.pricelist_id.id,
         })
         product = self.env['product.product'].with_context(product_context).browse(product_id)
-        pu = product.price
-        if order.pricelist_id and order.partner_id:
-            order_line = order._cart_find_product_line(product.id)
-            if order_line:
-                pu = self.env['account.tax']._fix_tax_included_price_company(pu, product.taxes_id, order_line[0].tax_id, self.company_id)
+        discount = 0
+
+        if order.pricelist_id.discount_policy == 'without_discount':
+            # This part is pretty much a copy-paste of the method '_onchange_discount' of
+            # 'sale.order.line'.
+            price, rule_id = order.pricelist_id.with_context(product_context).get_product_price_rule(product, qty or 1.0, order.partner_id)
+            pu, currency_id = request.env['sale.order.line'].with_context(product_context)._get_real_price_currency(product, rule_id, qty, product.uom_id, order.pricelist_id.id)
+            if pu != 0:
+                if order.pricelist_id.currency_id.id != currency_id:
+                    # we need new_list_price in the same currency as price, which is in the SO's pricelist's currency
+                    pu = request.env['res.currency'].browse(currency_id).with_context(product_context).compute(pu, order.pricelist_id.currency_id)
+                discount = (pu - price) / pu * 100
+                if discount < 0:
+                    discount = 0
+        else:
+            pu = product.price
+            if order.pricelist_id and order.partner_id:
+                order_line = order._cart_find_product_line(product.id)
+                if order_line:
+                    pu = self.env['account.tax']._fix_tax_included_price_company(pu, product.taxes_id, order_line[0].tax_id, self.company_id)
 
         return {
             'product_id': product_id,
@@ -110,6 +125,7 @@ class SaleOrder(models.Model):
             'order_id': order_id,
             'product_uom': product.uom_id.id,
             'price_unit': pu,
+            'discount': discount,
         }
 
     @api.multi