577 - Include all problems in overview

This change makes all problems appear in the overview. While 0 score problems are not relevant for the statistics of the exam, the statistics of the grading time, the amount of submissions graded, rubric item distributions, etc. are still very relevant, and are now displayed.

I have checked the calculations that are made to confirm that adding these problems has no impact on their calculations. Max score is unaffected, since these problems have zero max score. For cronbachs alpha, the variance will be unaffected since the variance of the problem is 0 (all students have the same score of 0). Full scores length is the number of students, also unaffected. For correlation scores, I have specifically added a requirement of having a non zero maximum score (to avoid a console warning), and the question is ignored for further calculation.

The fact that all questions, including those who don't have rubrics yet now appear in the overview table at the bottom is also an improvement I think, as it ensures that no questions are forgotten in the grading.

Closes #577