[IMP] website_sale_digital: downloadable attachment marked by product_downloadable field

This allows the user to continue using the attachment feature, without making it public by
default, which was the case in the previous implementation. If you wanted to use the
product digital attachments, you were foresaking the standard attachment feature since
everything you would attach to the product would then be public and downloadable.

The "digital" product type was also removed, because you could very well want to provide
downloads for a stockable product, like an online manual or a warranty document, or
provide downloads for a service, especially considering digital products are considered
service in european laws.

The product template and variants stat buttons have also been changed:
- The template stat button now only displays the files associated at the template level,
instead of showing all the files available accross all variants.
- The variant stat button now displays the files associated with it but also with the
corresponding template, instead of showing only the files associated with the variant.

These new stat buttons are closer to what the customer will see in practice, since the
variant level stat button will show the set of files that the customer will be able to
download when he has bought that product variant. Previously, the template showed a set
of files that was never presented to any customer (since it contained the files from all
the variants at once), and the variant was showing less files, since it was ignoring the
ones from the template.

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