Cleanup TinyMCE on Outcomes pages if it exists

Test Plan:

    - On the outcomes page, switch between two or more outcomes and
      observe that there are no JS errors (undefined type error)
    - edit an outcome, but click on a different outcome befor clicking
      either the cancel or submit buttons.  Observe no JS errors
    - edit an outcome.  Click cancel and then select a different
      outcome.  Observe no JS errors

Fixes CNVS-16825

Change-Id: I748b3e40b6e64eec78aa9c8ae2bd7eb708211486
Reviewed-on: https://gerrit.instructure.com/44283
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Adam Stone <astone@instructure.com>
Product-Review: Benjamin Porter <bporter@instructure.com>