Draft: fix filtering

1. We filter call events (because we don’t support VoIP yet) but only on initial sync, so we still get a notification count increase (the number bubble in the sidebar, not the actual notifications).
2. We use the filter only on initial sync and I was unable to find a valid reason for that.
3. After fixing 1. and 2. by using the (same) filter for all sync requests, I am now looking to create and store that filter on the server, so that I don’t pass it as parameter on every sync request. There is API for that.

#741 (closed) probably should be addressed as part of this too.