Regrade events do not show up in the Gradebook History

Regrade events are not shown in the gradebook history.  Adding this
feature will provide a simple and convenient way to see ALL grading
events in one place.

Fixes CNVS-8590

Test Plan:
  - Create a quiz event
  - Take the quiz with a student
  - Regrade the quiz as an instructor
  - Confirm that regrades are shown in the gradebook history

Change-Id: I3925f0fefaf80f0426427c774b0fc63835d1f317
Reviewed-on: https://gerrit.instructure.com/39977
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Ryan Taylor <rtaylor@instructure.com>