[UX] Set date formats by country

Spun off from https://github.com/backdrop/backdrop-issues/issues/1119 since not all of that issue is going to make it in to 1.3.0.

Can we add date format by country (https://en.wikipedia.org/wiki/Date_format_by_country) data to standard.inc?

Yes! I think this sounds like a great proposal. Let's work on it here :)

I also think this a great idea for the installer and for the site settings. But how would this work for individual users? If user A uses YYYY.MM.DD but user B uses MM/DD/YYYY, should this be accounted for based on their individual time zones? I'm not sure this is easily possible. Maybe we should just start with site-wide format and the automatic guessing based on timezone and work on per-user timezones later.

Food for thought....