Change the base branch of a Pull Request

Super exciting, as this mistake is SO OFTEN made, especially in open source projects (and definitely in a few internal projects I've been on).

Been missing out on this feature for a long while, especially on flows where you have had bug fixes pending for `x.x` that need to land somewhere else than on master first after you have submitted the PR (for example on a release branch on gitflow-ish branching models). And of course when you pick the wrong base by mistake too. Really glad it's there now!

It is really a needed feature, glad we have it now.