Correctly assign aria-expanded to checkboxes

aria-expanded attribute is handled
ui/boot/initializers/activateElementToggler.js

it will only add the aria-expanded to the controlling
element if it already has an aria-expanded attribute

$trigger && $trigger.attr('aria-expanded') !== undefined

closes VICE-3736
flag=none

Test Plan
1. open legacy create discussion
2. Click on Allow liking
3. Click on Enable podcast feed
4. Verify that those checkboxes have the
aria-expanded=true attribute not the option that
was revealed by the checkbox

Change-Id: I1a3f439567a5a1fa85735b311ab9013fab820360
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/338188
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Omar Soto-Fortuño <omar.soto@instructure.com>
Product-Review: Omar Soto-Fortuño <omar.soto@instructure.com>
QA-Review: Chawn Neal <chawn.neal@instructure.com>

ui/features/discussion_topic_edit/jst/EditView.handlebars

@@ -195,6 +195,7 @@
               {{checkbox "podcast_enabled"
               checked=podcast_url
               class="element_toggler"
+              aria-expanded=podcast_url
               aria-controls="podcast_has_student_posts_container"}}
               {{#t "enable_podcast_feed"}}Enable podcast feed{{/t}}
             </label></div>
@@ -225,6 +226,7 @@
               id="allow_rating"
               class="element_toggler"
               checked=allow_rating
+              aria-expanded=allow_rating
               aria-controls="rating_settings_container"}}
               {{#t "allow_liking"}}Allow liking{{/t}}
             </label></div>