Adds 'prev' and 'next' links on StudentExercise show page

This PR should partially address student feedback about navigation between StudentExercises.

'prev' and 'next' links now appear in the side nav area, below the numbered list of StudentExercise links.  If the first/last StudentExercise is being shown, the 'prev'/'last' link will not appear.  The links will take the user to the prev/next StudentExercise regardless of completion or feedback status.