change Gradebook cell input type to text

fixes CNVS-13247

- In Firefox when a user hit up on the first cell or down on the last cell,
  it would increment or decrement the respective number. This is the browser
  behavior for inputs with the number type, so the type was changed to text.

Test plan:
- In Firefox, open a gradebook, edit a top cell, and hit the up arrow.
  Confirm that the number retians its value.
- Then edit a bottom cell, and hit the down arrow. Confirm that number maintains
  its value.

Change-Id: I9149d5165c5a66cda63dc48f5c94497600c2fd18
Reviewed-on: https://gerrit.instructure.com/45449
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Reviewed-by: Spencer Olson <solson@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>