calendar/date/delete: added fieldset to decide what to do with dates that are...

calendar/date/delete: added fieldset to decide what to do with dates that are in multiple calendars, fixes #3882

Closes #3882

Merge request studip/studip!2739


(cherry picked from commit 7657df71)

22cfe5ac calendar/date/delete: added fieldset to decide what to do with dates that are in multiple calendars
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