Allow for creating feedback in the exam view

This merge request allows a user to add feedback to an exam in the /exam/ view. This is a prerequisite for grading multiple-choice questions, and might otherwise be a nice to have so course staff can create feedback before submissions are uploaded.

I re-used the feedback components used in the grading view. I added a grading option when creating a FeedbackPanel to indicate whether the checkboxes should be rendered in the feedback list. This also changes the behavior of clicking on an option: in grading clicking a feedback option (de-)activates it for a user, in the exam it brings up the edit panel for that option.

This closes works-on-my-machine/zesje#8 (closed)