Add "locale default" date/time format

So I think I figured out why the date/time stuff is annoying: there *is* a default format for each locale; you can confirm by changing your "Formats" in the Language & Region plug and observing time formats in the little preview. Other OSes (macOS and Android, at least) seem to just default to that time format ("locale default" or similar) but then also let a user override it in the time settings.

I think when people pick a region with certain formats, they expect the time format to change along with that—not to have to go into Date & Time settings to correct it.