add final grade override feature flag
add final grade override feature flag

closes GRADE-1584

Test Plan
- Go to the account settings for feature flags.
- Verify that there is a setting for Final Grade Override in the
  allowed state.
- Go to the course settings for feature flags.
- Verify that the course can toggle the feature flag for Final Grade
  Override on or off.

Change-Id: I107671b4f458cb0f9b73db4180d3b101c55cf29e
Reviewed-on: https://gerrit.instructure.com/169729
Tested-by: Jenkins
Reviewed-by: Adrian Packel <9c6a39807293376f29cba17542b4cb1b663ecc4a@instructure.com>
Reviewed-by: Keith Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>
QA-Review: Gary Mei <4dff5ebcff9ed76793b70b9470ac64791a16ed11@instructure.com>
Product-Review: Sidharth Oberoi <b9947257dd0ff1b6a5d899864f5ca846b60f0be3@instructure.com>

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