add_carousel_descriptions

New feature to return app descriptions to storefront-front carousel. The original descriptions were removed by Valve because they were too hard to translate and maintain; this makes an OK replacement.

Can be later improved if the small description/blurb under the header on the right of app pages gets exposed via the API (this uses the about_the_game field, which is in most cases too long); I'll put in a request for this field to be exposed.