Creation of 'on sale' price feature

[BethanOFN]

Description

This was requested by a user who wanted to be able to indicate when products were on sale, whilst also indicating the original price e.g. this example from another website

This is to indicate the saving to the customer and help to boost sales. Currently hubs rarely reduce the prices of items as the purpose of a sale is to attract attention and increase sales, which isn't possible with the current platform.

Expected Behavior

1. Enterprises should be able to add a 'sale price' from the edit product screen, separately to the original price
2. 
3. Once saved, the shopfront would then show the original price crossed out, with the sale price boldened next to it and an 'on sale' banner as in the example above.
4. Customers should be able to purchase the item at the sale price.
5. Reports and invoices should show the sale price.
6. To remove the sale price, enterprises should delete the sale price from the sale price field in edit product.

Severity

This might be a good candidate for hackathon/papercuts depending on development required

[sigmundpetersen]

Thank you @BethanOFN. This belongs in the Wishlist repo, so transferring there. It will be evaluated if it's papercut material or not in the monthly meeting.

[RachL]

Not a papercut as is :( The relationships with fees and tagging or with unit price is tough. this needs a @openfoodfoundation/train-drivers-product-owners inception.