& use in DocumentationBatchCreateOrUpdateMutation & AbsencesBatchCreateMutation.

& share between query & mutation.

& use documentations instead of lesson-events

into get_documentations_for_events
& get_for_coursebook
This makes it reusable for further purposes (absences).

& do the TODO.
Also leave a pointer how to improve it (even) more.

It does not have to be the currentDate. This holds only for the first day.

Updating the hash fragment causes a small delay while scrolling.
=> Update only after user finished scrolling.
Uses requestIdleCallback
which is not ready for prime time in Safari. But should work elsewhere.

by introducing rootMargin.

app-bar & crud-iterator header where not included in top element
calculation.
165 px is again a magic number & should be replaced by direct
references.
But it is still not clear how to get the app-bar height.