Regard teachers' working times when planning lessons
Depends on https://edugit.org/AlekSIS/onboarding/aleksis-app-kolego/-/issues/2
- Show planned working time of teacher when planning lessons in timetable raster frontend (like the planned lesson quota of the course is already shown) in clock hours (and not as a slot count!)
- Add working time statistics page showing actual working hours (aka the aggregated time of all lessons and supervisions of a teacher) and planned working times