allow grading scheme values with two decimal places
allow grading scheme values with two decimal places

fixes TALLY-55
flag = none

test plan:
 Note: There is an issue with editing a grading scheme
 on the course settings page more than once. It is a
 known issue on master, separate from this change.

 1. Visit the course settings page
 2. Ensure "Enable course grading scheme" is checked
 3. Click "view grading scheme"
 4. Edit the grading scheme
 5. Set a value with two decimal places
 6. Save the scheme
 7. Ensure the scheme preserves your change

Change-Id: I2cb414999ed6276bbcbf9ad9912de63da694554b
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/227051
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Nick Pitrak <1d88e258cca3cea49ddd3c88644b8ac147b565af@instructure.com>
Reviewed-by: Adrian Packel <9c6a39807293376f29cba17542b4cb1b663ecc4a@instructure.com>
QA-Review: Adrian Packel <9c6a39807293376f29cba17542b4cb1b663ecc4a@instructure.com>
Product-Review: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms