Add a review difference option between exercise variants in exam mode

[krusche]

When creating exercise variants in exams, it would be interesting to highlight the differences between multiple exercise variants, similar to reviewing changes in one exercise between template and solution repo.

This should ideally include all 3 repos and the problem statement for programming exercises.

A two variant diff should be relatively easy to implement, however it's unclear if more than 2 variants can be compared and visualized at the same time. In case it is only possible to compare two variants, the instructor must select which to compare with a selection in case there are more than 2 variants.

For other exercise types, a custom comparison should be developed, e.g. only comparing the problem statement for text and modeling. For quiz, the question content could be compared as well, however this would be a bit more difficult.

Ideally the comparison always highlight single words in case two lines in a problem statement are similar