[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.

To test:

* Set your user agent to a Firefox for Android UA.
* Go to http://localhost:8080/en-US/kb/how-sync-works-old-version-firefox?mobile=0
* The "Customize this Article" section in the sidebar should say "Firefox \ Version X \ Android"
* You should see numbered steps with no text.
* Apply the patch.
* Go to the same page.
* It should look fine now, and the selector should say "Firefox \ Version X \ Windows 8"

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