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
Yes, but this allows us to handle the expectations dictionary more easily (we can write `{tuple(json.loads(key)): val for key, val in expectations}`
on the one hand and on the other hand we can still access evaluations via idx as usual)