This commit lets you customize Livebook shutdown to gracefully power
off. One use case is for Nerves-based devices that need to do more than
call `System.stop/0` to power off.
This doesn't change the `LIVEBOOK_SHUTDOWN_ENABLED` environment
variable. It still works the same.
The `:shutdown_enabled` configuration is now `:shutdown_callback`. Valid
values are `nil` or an MFA. An unset or `nil` callback hides the
shutdown button in the UI.
