Updates StudentExercise and StudentExercise Feedback navigation

This PR expands on issue #274.

On the StudentExercise show page, there are now clear links to prev and next StudentExercises as well as a link to Feedback, if it is available.

On the StudentExercise feedback page, there are now clear links to next and prev Feedbacks, as well as a link to the StudentExercise associated with the currently displayed Feedback.

On both pages, there are links back to the Class and Assignment.  The spacing between links has been adjusted to form natural groups and to avoid visual clutter.