GitHub: require multiple reviews

Adds a build status that will not pass until N reviewers have approved
Readme:

Adds a GitHub check that will pass once the code has been reviewed by N people, where N is configurable (default: 2). You can combine this with protected branches to prevent merging until there have been N successful code reviews (and no changes requested).

Automatically invalidates the build status when new commits are pushed, requiring re-review.