input type set to text instead of number if browser is Safari

Safari does not fully support built-in HTML5 validations, error message was being skipped.

fixes CNVS-14162

test plan:
- Click the edit cog for the assignment group cog
- Enter text into min/max score fields
- Chrome and FF should display default HTML5 validation text, box should contain arrow incrementers
- Safari should display our built validation text "you must use a number" and arrow incrementers should be disabled

Change-Id: I8a25372a11de14e56288d42f68b0353a0b745414
Reviewed-on: https://gerrit.instructure.com/38450
Reviewed-by: Liz Abinante <labinante@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>