Add the ability to mark reviews as "read"/"done"

When you have large number of code reviews pending, it would be nice to be able to mark the reviews as "read" or "done" once you are finished with the code review and code review changes are required. When the PR is marked as "read", the author of the PR would then know that your review is complete and they can make the required changes. While the author is making the changes, the PR would not show up in the reviewers inbox until another commit is pushed. Currently, I have to manually keep track of which ones I've reviewed and keep checking them for new pushes that need to be reviewed.