Problem

A user may wish to receive notifications about a specific issue in a project, but not all issues.

Analysis

I don't think this one needs too much explanation, but it's essentially the same way in GitHub how you can "subscribe" to an issue. This is helpful so you don't have to leave a comment and generate noise if you want to keep current on a topic - you just click a button and you silently begin receiving notifications via email about new comments or discussion.

I also think it's helpful if the people "watching" an issue are publicly viewable as well. That way, if you're in a ticket and need someone's attention outside of the project, you could see if they are already receiving notifications in the public "watchlist" or if they are not receiving notifications yet.

I think this ticket could be an improvement to the wish explained in #746.

Implementation suggestion

1. Add button to sidebar of issues, e.g. "Watch issue", "Subscribe", "Follow discussion"
2. Clicking it adds you what is essentially the CC list for the ticket, but DOES NOT generate a comment or a public notification of you subscribing
3. Receive email updates about any traffic in the issue