discussions: fix "close for comments" text on reopened topics

fixes CNVS-12127

test plan:
  1. Log into a course as a teacher
  2. Create a discussion
  3. Navigate to the Discussion Index Page
  4. Close the discussion for comment
  5. Pin the discussion
  6. Open the cog menu and click on "Open for Comments"
  7. Open the cog menu again
  8. Verify that it gives you a "Close for Comments" option

Change-Id: I26036b0380beae75c79f893149099dfd572b71dd
Reviewed-on: https://gerrit.instructure.com/33048
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Clare Strong <clare@instructure.com>