Added VisibleCommentFeedbackSerializer

Only Comments that are `visible_to_student=True` will be serialized. For some weird reason i had to resort to a little hack in the serializer, see the comment inside the `get_feedback_lines()` method of the serializer for context.
I choose to not remove Feedback that is not final from the response of the student submissions endpoint (as outlined in #91) and will instead show a message in the frontend. This is easier to implement and potentially better for debugging in the frontend.