feat: Observe notification read and fix #6406 (#6407)

* Resolve https://github.com/syuilo/misskey/pull/6406#issuecomment-633203670



* Improve typing

* Observe notification read

* capture readAllNotifications

* fix

* fix

* Refactor

* Update src/client/components/notification.vue

Co-authored-by: syuilo <Syuilotan@yahoo.co.jp>

* Update src/client/components/notification.vue

Co-authored-by: syuilo <Syuilotan@yahoo.co.jp>

* missing ;

Co-authored-by: syuilo <Syuilotan@yahoo.co.jp>