If some future client ever sends us an arbitrary timer value which we do
not currently support, present it as a duration in seconds in timer
update messages and ui, where we would otherwise have rendered nothing,
e.g., "You set the timer to ."
// FREEBIE
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.
