Adding a UserData.LocalAccountInfo with a nil username while the
PreferencesView is on screen will cause a crash, since it triggers a
Combine publish upon which the PreferencesView expects to be able to
display the username of all accounts.
Onboarding view controller needs to set the account info object on the
mastodon controller before calling getOwnAccount since getOwnAccount
will upsert the user's account into the persistent container, which
requires the account info to exist to create a unique-per-account
identifier.
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