When initialized, or when expiration-related attributes change, expiring
messages will set timers to self-destruct. On self-destruct they trigger
'expired' events so that frontend listeners can clean up any collections
and views referencing them.
At startup, load all messages pending expiration so they can start their
timers even if they haven't been loaded in the frontend yet.
Todo: Remove expired conversation snippets from the left pane.
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