[FIX] website_sale: fix cart price discount calculation

To reproduce:
=============

- In the Settings: Product prices displayed on the website set as "Tax Included".
- Created a test product (for example price 100$ tax 15%).
- Created a price list with fixed price 15$ for all products
- Pricelist configured as "Selectable" and set to "Show public price & discount to the customer".
- on the product shop page, the price is 17.25$ instead of 115$ (which is the correct).
- add product to cart and check the cart price, it's 17.25$ instead of 100$.

Problem:
========

The undiscounted price displayed on the cart didn't consider the website
configuration (tax excluded/included).

Solution:
=========

Consider the website configuration to show the expected undiscounted
amount.

opw-3771242

closes odoo/odoo#157357

Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>
Co-authored-by: Victor Feyens <vfe@odoo.com>

addons/website_sale/models/sale_order_line.py

@@ -70,6 +70,14 @@ class SaleOrderLine(models.Model):
             self.shop_warning = ''
         return warn
 
+    def _get_displayed_unit_price(self):
+        show_tax = self.order_id.website_id.show_line_subtotals_tax_selection
+        tax_display = 'total_excluded' if show_tax == 'tax_excluded' else 'total_included'
+
+        return self.tax_id.compute_all(
+            self.price_unit, self.currency_id, 1, self.product_id, self.order_partner_id,
+        )[tax_display]
+
     def _show_in_cart(self):
         self.ensure_one()
         # Exclude delivery line from showing up in the cart

addons/website_sale/views/templates.xml

@@ -1605,7 +1605,7 @@
                             <t t-if="line.discount">
                                 <del t-attf-class="#{'text-danger mr8'}"
                                      style="white-space: nowrap;"
-                                     t-out="line.price_unit * line.product_uom_qty"
+                                     t-out="line._get_displayed_unit_price() * line.product_uom_qty"
                                      t-options="{'widget': 'monetary', 'display_currency': website.currency_id}"/>
                             </t>
                             <t t-if="website.show_line_subtotals_tax_selection == 'tax_excluded'"