DateTimeField must use datetime-local HTML input

[jace]

DateTimeField does not use the datetime-local HTML input type because it was poorly supported when the field was originally introduced.

According to CanIUse, the only laggards that don't support it now are Firefox and Safari desktop editions. Everyone else is on board. The UI on mobile is far better than anything we can cook up ourselves.

DateTimeField should start using datetime-local, with a polyfill for laggards. This polyfill works, although it could do with a better date widget than jQuery's.