Possible points server-side validation

fixes CNVS-7565

test plan:
- From Assignments screen, click 'Add Assignment'
in sidebar
- Click 'more options'
- On the resulting page, enter a negative number
for Points
- Submit the form
- You should see a message stating that the points
have to be a positive number

- From Assignments screen, click to edit an existing
assignment
- On the resulting page, enter a negative number
for Points
- Submit the form
- You should see a message stating that the points
have to be a positive number

Change-Id: I7e618dd8409c123e836ee3e192ad42cdf9d909d5
Reviewed-on: https://gerrit.instructure.com/33535
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>