Notification counts

This PR introduces some improved notification counting functionality. The aggregator will keep track whether a routed activity aggregated with an existing activity. When it does it will not be counted towards the unread count.

This does introduce a breaking change to the format of push notifications. Since we're now publishing on aggregation it's possible to have multiple activities.
Can we use something like https://github.com/wilmoore/selectn instead of rolling our own?