gb2.5 previous/next student and assignments

fixes CNVS-10018

test plan
- in a course with many students and assignments
- use the previous and next student/assignment buttons to navigate
  through dropdowns
- verify that 'previous' is disabled when there are no previous
  students/assignments
- verify that 'next' is disabled when there are no next
  students/assignments
- verify that everything works all hunky dory with screenreader
- try again in a smaller course
- change different sections and try navigating

Change-Id: I712763eb76b3957b7673b612a86b4331d996abca
Reviewed-on: https://gerrit.instructure.com/27818
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Matthew Irish <mirish@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Aaron Cannon <acannon@instructure.com>
Product-Review: Liz Abinante <labinante@instructure.com>