Add role="tablist" & aria-multiselectable="true" to accordion .panel-group to improve accessibility

Refs #13555.
Per https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#collapse
Credit: https://github.com/paypal/bootstrap-accessibility-plugin

[skip sauce]

docs/_includes/js/collapse.html

@@ -13,7 +13,7 @@
   <p>Using the collapse plugin, we built a simple accordion by extending the panel component.</p>
 
   <div class="bs-example">
-    <div class="panel-group" id="accordion">
+    <div class="panel-group" id="accordion" role="tablist" aria-multiselectable="true">
       <div class="panel panel-default">
         <div class="panel-heading">
           <h4 class="panel-title">
@@ -59,7 +59,7 @@
     </div>
   </div><!-- /example -->
 {% highlight html %}
-<div class="panel-group" id="accordion">
+<div class="panel-group" id="accordion" role="tablist" aria-multiselectable="true">
   <div class="panel panel-default">
     <div class="panel-heading">
       <h4 class="panel-title">