make auto_assigned peer reviews more robust

fixes CNVS-9494

test plan:
- NOTE: *** before checking out the commit ***
  - create an assignment with auto assigned peer reviews in the future
  - submit the assignment for two students
  - wait for auto assign time to pass, they should have peer reviews
  - export the course as an .imscc package

- create an assignment with auto assigned peer reviews in the future
- submit the assignment for two students
- wait for auto assign time to pass, they should have peer reviews

- now submit as two additional students, notice that they do not get
  auto_assigned
- move auto assign at time into the future, wait for it to pass
- they should now also have peer reviews

- import the package the was exported before checking out the commit
- it should import without errors
- submit the assignment as two students
- move auto assign at time into the future, wait for it to pass
- they should now also have peer reviews

Change-Id: I22316349c185c9f641152325fdcf536d585de49c
Reviewed-on: https://gerrit.instructure.com/39993
Reviewed-by: Benjamin Porter <bporter@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>