Add Abitur specific grades features

[Jonas-Sander]

For users in Abitur we could add specific functions to help them show their progress or setting a goal.

We could allow for users setting a specific Abitur goal grade (e.g. 1,2) and allow them to simulate what grades they will need to achieve to reach that goal. The predicition should include the already achieved grades. A user might then open a "simulation" where they can input specific grades for feature exams. Then we might calculate what grades they will have to achieve in the other exams / subjects.