event: Fix multiday detection for all-day events

All-day events always use UTC, so when the multiday detection code was
converting them to the local timezone, they span an additional day if
the local timezone is not the same as UTC. This was leading to all-day
events always being considered multiday events.

Fix this by ignoring the timezone for all-day events, like all other
comparisons do. Also don't consider all-day events spanning 1 day to be
multiday since as per RFC 5545 the end date is non-inclusive and if
omitted to be start + 1 day.
1 job for fix-all-day-multiday in 16 minutes and 34 seconds (queued for 1 second)