issue,posting: Add Notification for comment.

* A user can watch by clicking watch button on issue or posting.
* The author, assignee and commenters become watcher automatically.
* Watchers receive a notification email when someone leaves a comment
  on the posting.
* Watchers can stop watching by toggling watch button.

동작에 대한 설명은 https://github.com/npcode/hive/blob/feature/mail-notification-for-comments/docs/technical/watch.md 에 나와있습니다.