creates I18n for assignments with nil points possible

fixes CNVS-11025

test plan:
- in screenreader gradebook
- with an assignment with nil pts possible
- confirm that it says "No points possible"
- confirm that assignments with pts possible render the pts
- select a student
- confirm that it says "No points possible" next to the "Grade" box
- confirm that assignments with pts possible render correctly next to "Grade" box

Change-Id: Icddb54e2ca6fc5ac10138b76cb5a63999d6a6103
Reviewed-on: https://gerrit.instructure.com/29827
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Liz Abinante <labinante@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>