The instance timeline controller needs to store a strong reference to
the instance-specific MastodonController since the timeline VC only
holds a weak reference, and unlike normal screens, the scene session
doesn't hold onto the MastodonController for other instances.
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