The timestamp update work item shouldn't retain a reference to the cell.
It can be unowned because when the cell is deinit'd, the work item will
be cancelled.
dependency injection
The places still using the .shared property are cases where there is no
view controller from which to (easily) get the appropriate instance,
such as user activity and X-Callback-URL handling. These uses will need
to be revisited once there are multiple MastodonControllers.
See #16
Shadowfacts