Add "Next assignment" link for graders to move forward easily

It would be great to have a link to "Next assignment" for grading student work. At present you have to go back to the main "Assignments" list  (and scroll down) in order to select the next one. It's not a big deal, but it would be very convenient to have a link to pass directly to the next assignment's page--like the "Next lesson" link in the Lessons pages.