discussions: fix reply permissions disagreement

fixes CNVS-12950

test plan:

- create an unpublished discussion
- you should not see a reply textarea

Change-Id: Ib7dfc22df75f025c548f59bd5f3e3d18fa338cd2
Reviewed-on: https://gerrit.instructure.com/36249
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Drew Bowman <dbowman@instructure.com>