Provide partial evaluation results to admins and contestants

Implementation of https://github.com/cms-dev/cms/issues/343.

What's done:
- submission is being scored after each evaluation job completed;
- AWS always shows current evaluation results, so admins can see the progress of evaluation;
- CWS will show current evaluation results if an option "Show partial results" is set in task settings.

The only possible problem I see here is that a submission can now be "scored" before it was "evaluated". I hope I identified all the cases where "scored" actually means "evaluated" and "scored", but it's possible that I've missed something.