Add a warning when rebasing a PR that is not passing the tests

The same way we warn about merging an "unmergeable" PR, it will be ideal to have a warning if he PR is not passing the tests/build (should be possible to query the status API: https://developer.github.com/v3/repos/statuses/).
How generic is this functionality? Are all CI plugins going to affect PR metadata in this specific way or it is something only for Jenkins?