MR file browser should highlight current position

#14249 (closed) lead to the addition of a file browser on the left pane of the MR review interface. However, it would be more useful if the file browser would highlight the file currently in view.

Phabricator's Differential review interface has such a visual queue and I have found it remarkably useful for orienting myself within a patch (note that TcRnTypes.hs is highlighted in grey in the left pane):