add workflow_state to grading periods

closes CNVS-16215

Test plan:
  * create some grading periods
  * delete some
  * the api shouldn't return deleted grading periods in subsequent
    requests
  * the grading periods should be soft deleted

Change-Id: If4b795a05fd8eee9cde1bd90ab4db941004bc2a0
Reviewed-on: https://gerrit.instructure.com/43266
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>