DatePicker textfiled value should follow ISO 8601 by default

[mbrookes]

The US date format is ambiguous for dates with day <=12 && day !== month.

05/02/2016 could be read as either May 2nd, or Feb 5th.

I'm thinking ISO8601 (YYYY-MM-DD) would be a good default as it works everywhere, and doesn't have the localisation issues that moving to a short-month-name default (e.g. "2 May 2016") would bring.

Admittedly most users will want to change the date format through the provided hook, but a sensible default would be... sensible.

Happy to create a PR.

[oliviertassinari]

We have also started a discussion linked to it here #1658 (comment)

[mbrookes]

I saw that issue, but it was primarily about full i18n. This would be more of a quick fix for the default date format.

It needs to be fixed regardless of what approach is chosen for i18n, as there will always be a default, so it might as well be a sensible one.