Edit widget: allow to select whole weeks or months at once

To ease the task of updating the availability, it would be nice if an editor could select:
- a whole week at once (by clicking on the week number, that should thus be enabled in the widget settings before it can work)
- a whole month at once (by clicking on the month name)
- all same days of the week in a month (by clicking on one of the day of the week names)

As a side request: selecting a whole week (or even a whole month) could be implemented for the formatter as well, but that will take additional logic to determine what weeks and months are fully selectable (though that is currently also not checked).