Using Reviewable.io for Code Review

I can't recommend Reviewable highly enough! We've been using it for a long time now on Servo (https://github.com/servo/servo/). The author has been amazingly responsive with fixing bugs, dealing with GitHub issues, improving scalability on reviews with thousands of files or hundreds of thousands of lines of changes, making it work with really horrific rebase-heavy workflows, etc.

If you're using the GitHub code review UI today, reviewable is pretty much an immediate upgrade. We've found that we use it for anything non-trivial. So, if it's more than a few lines or would take more than a couple of comments, we use Reviewable instead.

Pretty much the only things we have issues with are:

- The UI is a little fancy. Sometimes figuring out how to get back to the PR itself so that I can tell our autolander service (http://homu.io/) to land the PR is hard because the dancing CSS butterflies get in the way :-)

- We're a Mozilla project, so we disproportionately use Firefox. Let's just say it's not the browser of primary testing (though it's ahead of some others!), given the demographic of developers who access GitHub.

- Every once in a while really long-lived PRs get rebased, squashed, and reordered so many times that all of the introduced pseudo-commits are bigger than the actual thing to review itself. I'm really not sure what could be done to help here, though.