Added validations for possible points field when grade type is letter or GPA

Added test for percent, letter, and GPA grade validation, updated tests

test plan:
- create a new assignment
- set points possible field to 0
- set display grade as letter grade, gpa scale, or percentage
- attempt to save assignment
- field should show "Points possible must be more than 0 for selected grading type"

fixes CNVS-4109

Change-Id: Ie4eb3fbfbf15433e61a8a3626b586c456919eff5
Reviewed-on: https://gerrit.instructure.com/38716
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>