Send announcement notifications on start date

[Gerzer]

Currently, announcement notifications are sent via APNS as soon as the announcement is submitted to the server, even if that announcement’s start date is in the future. We should instead schedule up a job that sends the notifications at the right time on the start date if that date is far enough in the future (an hour or longer, perhaps?).

[Gerzer]

We should also set the expiration date in the APNSAlertNotification structure to the announcement’s end date, taking into account its schedule type.