DA - srgb shows message if assignment is not visible

fixes CNVS-14136

test plan:
in a course with differentiated assignments enabled and set up
- view screenreader gradebook
- scroll through the assignment and student lists
- verify that a message appears indicating that an assignment is not visible
- verify that instructors can grade visible assignments

- grade an assignment for all students who can see it
- move a student with 1+ graded submissions from one section to another that does not have assignment visibility
- view screenreader gradebook
- verify that their previous graded submissions are visible (and still graded)
- verify that you can re-grade previously graded submissions

Change-Id: I55f93abe05a7aafac82001d5e3b4c88fa80d72cf
Reviewed-on: https://gerrit.instructure.com/38081
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Product-Review: Liz Abinante <labinante@instructure.com>