Display datetimes as ISO format

[jrtcppv]

Datetime widget currently is US style MM/DD/YYYY. To support international datetime presentations, switch to YYYY-MM-DD or something similarly uncontroversial.

[marktaipan-cvisionai]

One potential path forward is to set the default version of the datetime widget to text instead of the calendar picker since the text displayed is isoformat. I would suggest adding something to be read from the style field of the attribute type.