set focus for assignment muter dialogs

fixes CNVS-10555

test plan:
 In screenreader gradebook and gradebook2
  - open the assignment muter dialog
  - focus should be on the content of the dialog

Change-Id: Id4972ebef2f9a496b984bd0e799172844ffbb158
Reviewed-on: https://gerrit.instructure.com/29309
Reviewed-by: Matthew Irish <mirish@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>