Add an option to ignore a pull request

If you get added as a reviewer for a pull request, but you have nothing to say about it (you never work on that part of the codebase before and it is a massive refactoring, for example) you presently have two options: active (approve pull request w/o fully understanding it) or passive (ignore pull request until someone else approves it and it gets merged).

There should be a button for me to remove myself from the list of reviewers, making ignore step active.