fix discussion topic submission creation for locked assignments

test plan:
* create a non-graded discussion topic
* reply to the topic as a student
* edit the discussion to make it a graded assignment
 that is locked (i.e. available until a past date)
* submissions for the student's entries should be
 visible in speedgrader

closes #CNVS-8768

Change-Id: I1e93cb4d34138b4b37ac43681ba6d08eea89ec9b
Reviewed-on: https://gerrit.instructure.com/45461
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>