adds screenreader label for sidebar menu

fixes CNVS-11002

test plan
- inside of a course
- using a screenreader
- for the "Assignments", "Grades" and "Settings" left sidebar items
- verify they are each preceded with "Course" (ex: "Course Assignments")
- verify that menu does't change for sighted users

Change-Id: I727576bd70c49a3b0bff96bf40fcf5b6a40f95ba
Reviewed-on: https://gerrit.instructure.com/34907
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Liz Abinante <labinante@instructure.com>