Add support for draft changesets in reconciler
Created by: eseliger
This PR adds support for creating and undrafting PRs on GitHub to the reconciler. It is the main chunk of work missing towards https://github.com/sourcegraph/sourcegraph/issues/7998, before I can start GitLab support, and some minor UI/GQLAPI things.
I tried making the commits as descriptive as possible, and I would strongly suggest to review this PR commit-by-commit, all at their own are pretty simple to understand, but they are all three needed to form a working reconciler implementation that works, hence I made them be one PR, consisting of 3 pieces.