show unstable jobs over a yellow background

This fix was necessary because we really need to know when a job is marked as unstable due to post build actions, e.g., coverage report, open tasks, ... etc

With these changes unstable jobs are shown over a yellow background by applying a new css class named unstable.

Exception are the jobs with failing tests, those are still marked as failed. This exception exists because it was the old plugin behaviour.

The issue "Unstable 'yellow' builds fail to show up" should also by fixed by these