Add "empty" field to date and datetime widgets.

[drlogout]

- Summary

This PR adds an "empty" field to the date and datetime widgets. E.g. a list widget called events has a start and an end date widget. If empty: true is added to the end date widget, the user can handle the end date optionally.

...
fields:
  - name: events
     label: Events
     widget: list
     fields:
       - {label: "Name", name: "name", widget: "string"}
       - {label: "Start date", name: "start_date", widget: "date"}
       - {label: "End date", name: "end_date", widget: "date", empty: true}

[erquhart]

@drlogout sorry for the delay here - have you checked out @bruce-one's work in #645?

I would love your thoughts on that based on your use case. If you could try it out locally and provide a review it'd help get it merged.

[erquhart]

Closing in favor of #842.