R1:c5162074a12a

Fix an issue where internal Calendar DateTimes would not be correctly set to all-day

Summary:
Ref T11816. I don't really know what happened here, maybe I rewrote and broke this at the last second?

In most cases, we directly respect the `isAllDay` flag on the event, so the internal date state doesn't matter too much.

However, in the case of mail notifications, the raw internal state is relevant. This should fix mail notifications for all-day events.

(I might still turn them off since I'm not sure…