fix module completion requirements for graded discussions

test plan:
 0. have a course containing a graded discussion and a
    non-graded discussion
 1. with draft state off and on:
  a. add both discussions to a module, and do not reload the page
  b. edit the module and try to set completion requirements for
     each:
    - both discussions should show up in the "Discussions"
      category in the requirements list
    - the graded discussion should allow you to set
      "must submit the assignment" and "must score at least"
      requirements
    - the ungraded discussion should disable these
  c. reload the page and repeat the previous step

fixes CNVS-12925

Change-Id: Ic680b2764387817061f1218b242c030259c133ed
Reviewed-on: https://gerrit.instructure.com/34570
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>