Change reply icon to lock/unlock icon.
Change reply icon to lock/unlock icon.

Fixes COMMS-1034.

Test Plan:
* Have some discussions, both locked and unlocked, both pinned
  and unpinned.
* Notice the icon shows "unlock" if the discussion is locked
  and "locked" otherwise.
* Make sure it still works.

Change-Id: Ic6dc7b5b3b9ecf40063dbe16a3d6a7970a1e45d4
Reviewed-on: https://gerrit.instructure.com/145848
Tested-by: Jenkins
Reviewed-by: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>
QA-Review: Aaron Kc Hsu <089edc94045de3d9f5f372fb17706d21afcc259f@instructure.com>
Product-Review: Venk Natarajan <f9eab59b292314637a3c917fea4742741e36a19b@instructure.com>

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