[amp story shopping] Price discount / badge

### Description

Enable a slashed out price.

An `originalPrice` field could be added to the config. "price" seems like it should always be the active price.

Badge UX will need to be finalized. This could be a set list of strings like "discounted" or "two days only".

~~Look into possibility of updating this like amp-list, where it only updates a section of the data so it's always up-to-date.
This can be accomplished by using a `src` attribute and updating the data but there is interest in understanding if just a piece of the template can be updated.~~
The recommended approach for this is to use the `src` attribute instead of updating the inline JSON.