More accurate notification counts

When incrementing the notification count for a user we take into
account whether the activity aggregated with an older existing activity.
If the new activity aggregated with such an existing activity, we do not
increment the notification count.
Notifiation count follow-up:

- The push streams can now contain multiple activities
- Notifications are sent to the push streams after aggregation
- Fixed up failing unit tests
Leaky push fix
Notification count - Fix for multiple aggregates
Fixed some documentation in the activity aggregation