Aggregate notifications to prevent spam

[Fenrikur]

As it may happen that multiple changes to events as well as other announcements occur within a short time frame, it might be worth considering aggregating them into a single push notification instead of potentially spamming clients with notifications.
If more than X announcements are submitted to the backend within Y seconds, the backend should aggregate them into a single notification, informing users about multiple new announcements to be viewed in detail from within the actual app.

This is merely a suggestion and should probably not to be implemented for EF23.