filter assignments in addition to submissions for DueDateCacher

helps postgres avoid a temp table

Change-Id: I33ffd1f3d35523eb350a8adff1a54eff799c95da
Reviewed-on: https://gerrit.instructure.com/30297
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>