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>

app/coffeescripts/views/outcomes/OutcomeContentBase.coffee

@@ -121,6 +121,7 @@ define [
       @$('form').toJSON()
 
     remove: ->
+      @_cleanUpTiny() if @tinymceExists()
       @$el.hideErrors()
       @model.destroy() if @state is 'add' and @model.isNew()
       super arguments...
@@ -178,7 +179,7 @@ define [
     # Called from subclasses in render.
     readyForm: ->
       setTimeout =>
-        @$('textarea').editorBox() # tinymce
+        @$('textarea').editorBox() # tinymce initializer
         @setupTinyMCEViewSwitcher()
         @addTinyMCEKeyboardShortcuts()
         @$('input:first').focus()
@@ -188,3 +189,6 @@ define [
 
     updateTitle: (e) =>
       @model.set 'title', e.currentTarget.value
+
+    tinymceExists: =>
+      return @$el.find('[name="description"]').length > 0 and @$el.find('[name="description"]').editorBox('exists?')