Years ago we started to implement a way to schedule a notification. We hit a problem but we never came up with a good solution and the feature never made it back to the top of the prioriry list.
This PR removes the code for scheduled_for. There will be another PR to drop the scheduled_notifications table.
Unforetunately, I don't think we can remove the `scheduled_for` attribute from the notification.serialized method because out clients might fail if something is missing. For now I have left it in but defaulted the value to None.
003ae7521b