fix(notifications): make scheduler send own group's notifications only

To fix the bug Ryan observed where four identical notifications were sent to the same user for the same channel. This was because the sendNotifications function, called by each group's scheduler, was sending notifications for all events in the notification_digest table, regardless of which group they belonged to. So each of the four schedulers sent identical notifications at about the same time.

The solution was to only send notifications for the group the scheduler belongs to.