Mastodon allows the user to configure a default visibility which should
apply to all clients. This setting is returned by the
/api/v1/preferences method.
Fetch the user preferences when the TUI starts, and use it to set the
default visibility when composing a new toot. The preference can be
overridden by a new command-line option, toot tui --default-visibility=.
If neither the preference nor the option are set, fall back to
get_default_visibility().
