Watch file for changes/email notification on file change

Implement a watch file feature that will send the user an email when the file changes. Imagine you keep important documentation in a repository and you want to know when a specific file changes.

Originally at http://feedback.gitlab.com/forums/176466-deprecated-feedback-forum/suggestions/6584124-watch-file-for-changes-email-notification-on-file & https://github.com/gitlabhq/gitlabhq/issues/2536