sets step value of default_grade input to allow decimals.
sets step value of default_grade input to allow decimals.

Fixes CNVS-4804

Test plan:
- In Chrome, select assignment in Gradebook2
- In the column header drop down, select "Set default grade"
- Enter a value such as 1.1
- No validation error should appear.

Change-Id: Ic2e43eb9135f782dc9e2c96f81aafed4efca4611
Reviewed-on: https://gerrit.instructure.com/33641
Reviewed-by: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Amber Taniuchi <ef4bfd1e6d278f6bc7d4a478f02be2cb9581a0a5@instructure.com>
Product-Review: Coraline Ehmke <175b95f02a0f719e66a95dc1a3fbc16c9eff6ab1@instructure.com>

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