make 'open for comments' wording consistent
make 'open for comments' wording consistent

fixes CNVS-8924

test plan:
- go to the discussion index page
- click a discussion cog and choose 'Close for comments'
- click the cog again
- the wording should now be 'Open for comments'

Change-Id: I0d4bfb76c0576629329934c1e70ad87502ed6ec3
Reviewed-on: https://gerrit.instructure.com/48399
Reviewed-by: Mike Nomitch <74443f6f110b6b7b934c89126258ed3d843d673f@instructure.com>
Tested-by: Jenkins
QA-Review: Adam Stone <a0b2b61e2edcf99aa16080c2254a6ae6923d1fac@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

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