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
Not sure how well this plays together with the rest of your code, but can't we just let the unique_name of a test case equal its codename (the string; not the tuple consisting of the codename). So the unique_name would be a tuple for tasks, subtasks and groups and a string for test cases.