Checks whether a user is still logged in, for the heartbeat.

Send a result that shows a log-in box if the user is no longer logged in,
or if their cookie is within the grace period.

function wp_auth_check( $response ) {
	$response['wp-auth-check'] = is_user_logged_in() && empty( $GLOBALS['login_grace_period'] );
	return $response;
}