fake submissions: make accepted_at fall back to submitted_at
fake submissions: make accepted_at fall back to submitted_at

test plan:
- specs pass

Change-Id: Ia7aaec969ae670cd7e7c4160bd0522bce051d8e5
Reviewed-on: https://gerrit.instructure.com/110727
Tested-by: Jenkins
Reviewed-by: Neil Gupta <cc4fec8a34766005dd45d48be045c08d736c11ed@instructure.com>
Product-Review: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>
QA-Review: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>

instructure/analytics,instructure/analytics,instructure/analytics