Make `uniqueId` optional

[smeijer]

I don't think it happens very often that a snackbar is closed based on some logic other than user interaction or timeout. So, it would be nice if the uniqueId could be made optional:

dispatch(showSnack('my-unique-id', {
  label: 'saved',
}));

dispatch(showSnack({
  label: 'saved',
}));```