add created_at and updated_at to assignment select columns

needed for assignment overrides

Change-Id: I663469270959ea70bb808ebe2d31384fe17bc611
Reviewed-on: https://gerrit.instructure.com/30870
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>