srgb global options: show concluded enrollments

fixes CNVS-10210

test plan:
 - in screen reader gradebook
 - check the 'Show Concluded Enrollments' checkbox
 - you should see students from concluded
   enrollments in the student drop down list
 - uncheck the checkbox and those students
   should no longer be there

Change-Id: I1bef2c484ce033494a3d1c8f0c1f394abcb6f34b
Reviewed-on: https://gerrit.instructure.com/28315
Reviewed-by: Matthew Irish <mirish@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>