treat ungraded as 0 in screenreader gb

adds `includeUngradedAssignments` as a controller property that is bound
to a checkbox

fixes CNVS-10211

test plan
=========
- navigate to screenreader gradebook
- check the "Treat Ungraded As 0" checkbox
- reload the page and ensure that it remains checked
- load gb2, and confirm that the setting is the same in the cog
  menu as it is in screenreader gradebook

Change-Id: I819b0754310c741fb7f81817a8cc92403385ff9e
Reviewed-on: https://gerrit.instructure.com/28139
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Matthew Irish <mirish@instructure.com>