Reset grade input field on tab restore
Reset grade input field on tab restore

Test plan:
  - In Chrome, Safari, and Firefox
    - Create an assignment and student
    - Open the assignment in SpeedGrader
    - Open a second tab (it can be a blank tab)
    - In the SpeedGrade tab, enter a grade
      - Maintain your focus in the input field
      - Do not blur; do not click away
    - With your keyboard, close the current tab
      - Restore the closed tab
    - The previously entered grade should not appear

Refs EVAL-435

flag=none

Change-Id: Ica8f0b9f14d2ee090d828e4051f918cd7f2f11f6
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/284359
QA-Review: Dustin Cowles <25afa0fec7fcf03720748195b533ef56026fd0cf@instructure.com>
Product-Review: Jody Sailor
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Eduardo Escobar <7e82d09bba767b0dd1222915e3d92141089e94a8@instructure.com>
Reviewed-by: Kai Bjorkman <4693b3d9e71784d629732770ab5e83ed8fbdb83e@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms