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
Ew! In the worst case, we could still put this whole code block into a python function (defined in a normal python file) and call that from here. Also somewhat ugly, but at least with sub-quadratic running time... ;)