calendar: Remove timezone offset from all-day events

All-day events always use UTC and aren't localized using timezones, their start and end timestamps are always at 00:00. Since javascript automatically applies the current timezone when using Date(), we will end up comparing a time like 05:00 if the user is in UTC+5 to a day (starting at 00:00 and ending at 23:59 UTC+5) inside _dateIntervalsOverlap, which leads to the event overlapping with two days.

This broke with 28c535e3, before that we somewhat accidentally removed the offset of the users timezone from the timestamp inside the calendar-server.

Fixes #1895 (closed)