add index on assignment_overrides.assignment_id

Change-Id: I55fde0b925c8ba9871723ab00e60870fa34716e3
Reviewed-on: https://gerrit.instructure.com/33945
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>