fixed conference collapse/expanding reading

Fixed the way screen reader reads the collapsed
and expanding of the new conferences and concluded
conferences reading.

fixes CNVS-21775

test plan:
- Go to the conferences page
- with screen reader on navigate to the
  new conferences header
- collapse and expand it notice the correct reading
- repeat above for concluded conferences
- repeat above for all screenreaders

Change-Id: I72dd37aef3410435b8326c1c99736889c8bad268
Reviewed-on: https://gerrit.instructure.com/59530
Reviewed-by: Jonathan Featherstone <jfeatherstone@instructure.com>
Tested-by: Jenkins
QA-Review: Heath Hales <hhales@instructure.com>
Product-Review: Peyton Craighill <pcraighill@instructure.com>

app/views/conferences/index.html.erb

@@ -33,7 +33,10 @@
   <div class="item-group-condensed">
     <div class="ig-header">
       <h2 class="ig-header-title">
-        <a class="element_toggler"
+        <a
+          class="element_toggler accessible-toggler"
+           href="#"
+           aria-label= "<%= t('New Conferences') %>"
            aria-controls="new-conference-list"
            aria-expanded="true">
           <i class="icon-mini-arrow-down auto_rotate"></i>
@@ -47,7 +50,10 @@
   <div class="item-group-condensed">
     <div class="ig-header">
       <h2 class="ig-header-title">
-        <a class="element_toggler"
+        <a
+            class="element_toggler accessible-toggler"
+            href="#"
+            aria-label= "<%= t('Concluded Conferences') %>"
             aria-controls="concluded-conference-list"
             aria-expanded="true">
           <i class="icon-mini-arrow-down auto_rotate"></i>