[19.0][ADD] website_sale_product_compare_price: show variant compare price, discount badge and saved amount

[idris-domatix]

19.0 [ADD]

Adds the website_sale_product_compare_price module.

On the product page, when the effective price of a product is lower than its
reference price, the module shows the amount the shopper saves:

• a crossed-out reference price,
• a discount badge with the percentage, and
• the saved amount in the website currency.

Two references are supported, following the native website_sale behaviour:

• the sales price of the variant (list price + price extra), when a pricelist
  rule that is not shown natively (e.g. a fixed price per product or variant)
  lowers the effective price;
• the manual Compare to Price field of the product template.

The block is only rendered when the native Comparison Price feature of the
e-commerce is enabled, keeps the native pricelist discount strikethrough
untouched, and stays in sync when the shopper selects another variant.

Tests are provided through MockRequest (combination info) plus an HttpCase
and a frontend tour.

@OCA/e-commerce-maintainers please review.

[idris-domatix]

Friendly ping for review. New 19.0 module (compare price, discount badge and saved amount); CI is green. @OCA/e-commerce-maintainers could someone take a look when you have a moment?