If a status in a conversation view controller creates a work item to
update the timestamp in 1 minute, but the view controller is deinit'd
before that time elapses, the mastodonController instance will be nil,
resulting in a crash.
The DispatchWorkItems's are cancelled by the respective cell deinit
methods. But if the work item has already begun, cancelling it has no
effect, potentially leading to a crash in the conditions described above
are true. Using a weak reference to self fixes this.
Additionally, don't unnecessarily recreate the work items every time.
They don't capture any local variables, only self, so nothing changes.
Shadowfacts
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