srgb - mute/unmute assignments

also clean up some specs

fixes CNVS-10022

test plan
- in screenreader gradebook
- pull up some muted assignments, unmuted them
- pull up some unmuted assignments, mute them
- checkbox should update to reflect muted state
- aria-label should update to reflect muted state
- database should save new muted state

Change-Id: Ic5b9c189b654bf387a983b54dcd8b6cb9728b3ec
Reviewed-on: https://gerrit.instructure.com/27895
Reviewed-by: Matthew Irish <mirish@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Liz Abinante <labinante@instructure.com>