settings/calendar: check for start and end time and allow the end time to be...

settings/calendar: check for start and end time and allow the end time to be set to 24:00, fixes #3868

Closes #3868

Merge request studip/studip!2721


(cherry picked from commit 833fdebf)

ff9275f7 settings/calendar: check for start and end time and allow the end time to be set to 24:00