Add info for Focus

[calixteman]

Since Focus is now available in crash-stats.mozilla.com as a product, it'd be useful to have build info on buildhub.

[peterbe]

@luser I'd love to not have to do this feature request in the old Buildhub, but instead, do it as part of Buildhub2.

The only thing holding us back from squaring away Buildhub2 is the l10n repacks. We have a Stage server now but I haven't backfilled it. Buildhub2 is much easier to update and maintain (since it's all new code and is easy to run locally).