Hide new puzzle notifications when puzzle is made private (rather than deleting)

[mdirolf]

When a public puzzle is made private we delete new puzzle notifications for obvious reasons. They aren't recreated when it's made public again to prevent double notifying when the setting is toggled.

Instead we should set some sort of flag on the notification when the puzzle is made private which we can unset when it's made public again. A simple hack might be to treat private as "private until" the max timestamp value.