fix intermittent RCE hyperlink bug

closes CNVS-12164

test plan:
  - visit an assignment show page
  - click edit and then within the RCE verify:
    - html link button present and working
    - html un-link button present and working
  - verify that all other RCE buttons continue working as before
    - including:
      - kaltura media
      - equella
      - LTI RCE integrations (youtube, khan academy, etc)
      - and all other default RCE buttons

  - NOTE: previous error was intermittent:
    - please make sure to go into and out of an assignment edit screen
      several times to verify that this has been fixed.
    - this bug seems to show up mostly in assignments, but testing in
      any other places where the RCE lives is advisable

Change-Id: Ie5dad019bb45dc0fc71d6e225a679b28b001a295
Reviewed-on: https://gerrit.instructure.com/33227
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jason Madsen <jmadsen@instructure.com>