The web app doesn't need any presence data for its first ping to
the server, because it already has up-to-date presence info in
page_params. So now we can tell the server not to send us a big
payload that we were already ignoring.
- Add `OFFLINE_THRESHOLD_SECS` settings parameter
to handle offline period.
- Set aggregated status to offline if user's status
haven't changed for `OFFLINE_THRESHOLD_SECS` period.
- Add test for offline aggregated status.
- Add aggregated status to user presence status dict.
- Add tests for aggregated presence status.
- Fix removing unused keys from status dict
with aggregated data for user.
Fixes#3692