include concluded enrollments in gradebook csv

closes CNVS-1296

Test plan:
  * conclude an enrollment in a course
  * open the gradebook, the user shouldn't be displayed
  * download the csv, the user should not be included
  * include concluded enrollments in gradebook display
  * download the csv again, it should include the user

Change-Id: I5a464b3fa5e963e9989837a216fd2732fc9979b6
Reviewed-on: https://gerrit.instructure.com/38954
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>