Implement SameSite cookie policy

- 세션 쿠키, 세션 보안키 등에 SameSite 속성을 적용할 수 있는 기능 추가
  (시스템 설정 -> 보안 설정)
- 일반적인 사이트는 Lax를 권장함, PG사 연동 오류 등의 경우 None 사용
- None 사용시 크롬 80부터는 SSL 전용 세션으로 지정해야 함
- Rhymix\Framework\Session에서 쿠키 관련 루틴들 정리
- PHP 7.3 이상, 7.2 이하 버전으로 나누어 처리

modules/admin/lang/en.php

@@ -164,6 +164,9 @@ $lang->use_server_push = 'Use HTTP/2 Server Push';
 $lang->use_gzip = 'gzip Compression';
 $lang->delay_session = 'Delay session start';
 $lang->about_delay_session = 'To improve performance when using a caching proxy server such as Varnish, do not issue sessions to visitors until they log in.<br>Selecting this option may interfere with autologin, and visitor counts may become inaccurate.';
+$lang->use_samesite = 'SameSite attribute';
+$lang->use_samesite_empty = 'Do not use';
+$lang->about_use_samesite = 'Set the SameSite attribute for session cookies and session keys.<br>Lax is the recommended setting for most sites. You may need to use None if you are having difficulties integrating with external services such as payment gateways.<br>However, None is only valid when used with SSL-only sessions.';
 $lang->use_session_keys = 'Use session security keys';
 $lang->about_use_session_keys = 'Use additional security keys to guard against session theft. This setting is highly recommended if you don\'t use SSL-only sessions.<br>This setting may cause some users to become logged out.';
 $lang->use_session_ssl = 'Use SSL-only session';