fix bulk_load_versioned_attachments bug

fixes CNVS-14509

previously it didn't include a submissions's attachment (used for
screenshots on url submissions)

Test plan:
  * make a url submission
  * use the submissions api (for_students endpoint) to get the submission
  * it should include an attachment for the screenshot of the submission

Change-Id: Ic4c2cfe693e0446bb48c811e8d2c5fd44e53d6a9
Reviewed-on: https://gerrit.instructure.com/37731
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>