Register student to a retake exam

[Daníel Sigurgeirsson]

It should be possible to register a student to a given retake exam.

(Implementation details: the code should obviously ensure the student is registered in the course, ensure both the course and the student exist, etc.)

The API should probably be:

/api/v1/courses/{id}/retake GET - returns a list of all students registered to the retake exam for the given course POST - adds a student to the retate exam (accepts the student SSN and possibly the type of registration: due to sickness or due to failing)

[Daníel Sigurgeirsson]

Finished in a recent PR