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
Maybe you could add a function ``unique_name`` to each of the following classes:
- ``MySubtask``: returns subtaskname
- ``MyGroup``: returns subtaskname.groupname
- ``MyCase``: returns casecodename