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
I think this doesn't work the way it is intended since the name of a group needn't be unique. In fact, the default name of the first group in *each* subtask is g0.