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.