Add begin/end code review to pull requests

I like in GitHub how you can say that you’re beginning a pull request, make all your comments, and only submit them once you’re done reviewing. With Bitbucket Server, the author gets an email notification about each individual comment as I make it, which prompted my teammates to turn on delayed notifications, so now they don’t get notified in as timely a manner.

I often want to revise an earlier comment once I finish reviewing, but the author has already seen the earlier comment, and may start replying to earlier ones before I’m done. This is all solved with a sort of “draft” state for PR reviews.