What version are you running?
Current rbcommons.com website

What's the URL of the page this enhancement relates to, if any?
Any active review, e.g., https://rbcommons.com/s/bti/r/66263/

Describe the enhancement and the motivation for it.
I would like the system to provide some visibility into which reviewers have opened/viewed a review.  
I often find myself attaching some reviewers to a review and then I'm left wondering if they have actually started the review or not.  After a time, I ping them on Slack to make sure they saw my review request.  Sometimes I find that they have not, and that prompts them to start the review.  Sometimes they are alread in the process of reviewing, and I have bothered them needlessly.
It would be nice if Review Board gave some indication of what people have actually opened the review on rbcommons.com and spent some time looking at it.

What operating system are you using? What browser?
macOS, Chrome

Please provide any additional information below.