Fix/improve how we handle all-day events across views.

All-day events always last to 0:00 now.

Bug: 12627
Fix saving all day events when the end date is on the next day

Internally horde uses two kinds of all day event formats:
- end on "current-day 23:59:59" (old)
- end on "next-day 00:00:00"

Properly detect "next-day 00:00:00" when doing the writeout.

Also switch to "next-day 00:00:00" format
when loading events for consistency.

Part of bug 12627

Signed-off-by: Jan Schneider <jan@horde.org>