Comment @mention, notifications, and unread list

[hoo761]

The press release promises that the Commenting feature will allow (among other things) users to "ask questions" and "delegate tasks" and "request edits". To do this, we'll likely need a system for mentioning people in a comment so that they are immediately notified of the new comment. Other platforms like Google Docs, Dropbox Paper, and GitHub have this same feature for the same reasons.

To fully delivery on the implied functionality described by the press release, we'll need to work on this.