Added school return management

From today it is possible to manage returns directly in WeekTable.

Returns are managed as fixed hours but outside of the standard timetable.
They can be activated per plex and then specified in which classes, on which days, and how many return hours are scheduled during the week.

At this point, you can assign individual hours to the teachers concerned, which will be deducted from the weekly hours.

All previews, exports, and sharing of the weekly timetable show any return hours.