[939781] Flip more switches for Thunderbird.
[Bug 968904] Showfor: Split platforms by product.

Before, all platforms were considered equal for things like filling in
select boxes, which causes weird things like forcing "Firefox for
Desktop on Android" if you visit a Firefox site on Android, which can
break a bunch of article content.

To fix that, this commit splits out the platforms by product, so that
the select box filler can behave more intelligently.