Add option to disable presence tracking for offline devices.

[sklamt]

This pull request adds the option offline_untracked to the presence.enabled configuration. This prevents synapse from updating the last_active_ts when receiving events from a client which is currently offline.

This would improve privacy by enabling users to decide whether they want their activity seen by other users with a shared room.

Pull Request Checklist

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[anoadragon453]

@sklamt I've approved CI to run. Note that someone doing so for you won't be necessary after one of your PRs are merged 🙂

The Matrix spec does not forbid this tweak, which is great. I think being able to disable informing other users about a user's activity while offline is a worthwhile feature. Doing so as an option is a good idea, unless it breaks previous assumptions.

[clokep]

Should it always do this? Is this a privacy leak? (Do we need an option?)

[sklamt]

I could also do this without an option. For us, changing the default behaviour would be fine.
The spec does not explicitly say anything about this, but I am not sure whether this may break some use cases.