I need a switch to Admin area which enables/disables crisis-mode. Redirect all traffic to certain page

Functionality needed:
There is natural disaster or terrorist attack etc. Admin login to site and enables "crisis mode". All traffic gets redirected to Crisis page.

I found this code from another answer, so I need the switch to admin area or admin menu which executes the code.

add_action( 'template_redirect', function() {
    if ( is_page( 813 ) ) {
        return;
    }

    wp_redirect( esc_url_raw( home_url( 'index.php?page_id=183' ) ) );
    exit;
} );