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/).
It should be generic, this should be what actually determines if the MERGE button is green or not in the web interface.