course progress API fix for deleted/unpublished module items

fixes CNVS-12769
fixes CAT-157

Test plan:
  1. Create a module with a few module items and set up
     completion requirements for them. Also be sure to require
     sequential progress.
  2. Hit the course progress API for that course and verify
     that the next_requirement_url points to the first module
     item.
  3. Delete the first module item.
  4. Hit the API again and verify that the next_requirement_url
     points to the next module item, not the one that was deleted.

Change-Id: If77ed034f897507f45b15e623fdbe2c1c8ab9208
Reviewed-on: https://gerrit.instructure.com/34082
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Landon Wilkins <lwilkins@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>