When selecting a start datetime, the end datetime should be after the start datetime.

[stevepiercy]

• When a user enters a start datetime in the widget for an event content type, its end datetime should update to be after the start datetime. In other words, the event duration must be a positive number.
• If the user enters an end datetime equal to or before the start datetime, then validation on both the frontend and backend will prevent saving the event and return a helpful error message: "End datetime must be after start datetime".
• For all-day events, the start time shall be 00:00:00 and end time 23:59:59 in the events timezone.
• For recurring events, an end datetime is optional.

Pulled from #5551.

[davisagli]

@stevepiercy Is this about validation, or about how the widgets operate, or something else?

[stevepiercy]

@davisagli, all the above. I updated the description to be explicit. @tisto as original proposer, and both @plone/volto-team and @plone/volto-accessibility, should review the revised issue description, as this is a pretty major change and improvement.