Fix compatibility with latest versions of DateTimePicker not showing

time panel

- Closes #2983
  It seems the `format` prop is required to have `HH:mm` to display the
timepanel…for some reason

- Note: The `first-day-of-week` seems unused, it should probably
  override the setting in `lang`
- Remove obsolete props `not-before` and `not-after` which are replaced
  by a `disabled-date` function prop in vue2-datepicker 3.0.0

Signed-off-by: Thomas Citharel <tcit@tcit.fr>