Adds any domain in a multisite installation for safe HTTP requests to the
allowed list.

Attached to the {@see ‘http_request_host_is_external’} filter.

function ms_allowed_http_request_hosts( $is_external, $host ) {
	global $wpdb;
	static $queried = array();
	if ( $is_external ) {
		return $is_external;
	}
	if ( get_network()->domain === $host ) {
		return true;
	}
	if ( isset( $queried[ $host ] ) ) {
		return $queried[ $host ];
	}
	$queried[ $host ] = (bool) $wpdb->get_var( $wpdb->prepare( "SELECT domain FROM $wpdb->blogs WHERE domain = %s LIMIT 1", $host ) );
	return $queried[ $host ];
}