Feedback and Unit Tests

* Feedback: table columns shown on "submissions" page depend on feedback
* Unit Tests:
    * changed semantics
    * implemented server-sided unit tests
    * seperate unit tests from normal submissions
    * special "details" page
How about ``public = self.submission_info.get("expected_public_score", public)`` instead of the try...except block? Or  specify the exception type, please.