allow account admins to view initial_post locked subtopics

test plan:
* have a discussion topic set to a group category
 with "Users must post before seeing replies" set
* as an account admin, should be able to view replies
 for a group subtopic without posting

closes #CNVS-39850

Change-Id: I92c284c3bc0d956a9c66d622022d4d1ad07c9224
Reviewed-on: https://gerrit.instructure.com/129028


Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Deepeeca Soundarrajan <dsoundarrajan@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>