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
Also, copy the state between screens, so e.g. expanding a status in the
timeline and then opening that conversation already has that status
expanded.
This intentionally doesn't store the sensitive attachment visibility
state, since showing text when not necessary is less dangerous than for
images. (Possibly a preference for this in the future?)
Closes#55
