[Wishlist] Make a way to track progress during code review (like "Viewed" checkboxes on Github)

Github has somewhat recently added the ability to make files as "viewed" during code review. This is a game changer, as it makes it much easier to track progress during larger code reviews. Note that the "viewed" state of each file is persisted on the server (at least as far as I can tell). It would be nice if Gitlab had a competitive offering in this area that helps reviewers deal with large reviews.