announce selected student, assignment for srgb

fixes CNVS-10369

test plan
- in screenreader gradebook
- use the dropdowns to select students, assignments
- verify that screenreader/JAWS/etc announces the selected
  student/assignment
- use prev/next buttons to select students, assignments
- verify correct text is read
- in VO buttons will announce that that they are dimmed when you try to
  click with last/first student/assignment selected, in JAWS it should
  be announced as "unavailable" when navigating over it
- verify when clicking, that assignment / student name is appended after
  the last item in the page (this will be visually hidden a separate commit)

Change-Id: I83f9447256777947c14641f04578a69d6f2dffe7
Reviewed-on: https://gerrit.instructure.com/29045
Reviewed-by: Liz Abinante <labinante@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>