use the current time for grade change audit events

fixes CNVS-12400

test plan: Enable auditors and trigger some grade change events, such as
auto-graded quiz or teacher-graded submission. They should show up in
the audit logs with the timestamp of the time the event happened.

Change-Id: I596242e09d8acb34443ca0974b7393fc9faa51f6
Reviewed-on: https://gerrit.instructure.com/33342
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>