Make exam a many to many field on StudentInfo model

Description / Overview

In the current stage, a student can only be assigned to one exam. Sometimes it is useful to allow a student to be assigned to multiple exams at the same time.

Use cases

Primarily useful for exercise correction. The reviewers can more dynamically decide how they want to organize their exercise grading. Some prefer to grade every week independently, while others want to grade all weeks as a whole. Currently, only the latter is possible. We could also reduce organizational overhead by providing one "exercise instance" for all exercises per semester. This way, there is less confusion on the student's side, as they currently have to switch between instances to see their exercises' scores for different modules.