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

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
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