Fix accordion example (#2823)

* make collapsible heading behave like a block
* remove the non-existent `panel-collapse` class
* replace the `in` class with `show`

pages/UI/general.html

@@ -1322,13 +1322,13 @@
                   <!-- we are adding the .class so bootstrap.js collapse plugin detects it -->
                   <div class="card card-primary">
                     <div class="card-header">
-                      <h4 class="card-title">
-                        <a data-toggle="collapse" data-parent="#accordion" href="#collapseOne">
+                      <h4 class="card-title w-100">
+                        <a class="d-block w-100" data-toggle="collapse" href="#collapseOne">
                           Collapsible Group Item #1
                         </a>
                       </h4>
                     </div>
-                    <div id="collapseOne" class="panel-collapse collapse in">
+                    <div id="collapseOne" class="collapse show" data-parent="#accordion">
                       <div class="card-body">
                         Anim pariatur cliche reprehenderit, enim eiusmod high life accusamus terry richardson ad squid.
                         3
@@ -1346,13 +1346,13 @@
                   </div>
                   <div class="card card-danger">
                     <div class="card-header">
-                      <h4 class="card-title">
-                        <a data-toggle="collapse" data-parent="#accordion" href="#collapseTwo">
+                      <h4 class="card-title w-100">
+                        <a class="d-block w-100" data-toggle="collapse" href="#collapseTwo">
                           Collapsible Group Danger
                         </a>
                       </h4>
                     </div>
-                    <div id="collapseTwo" class="panel-collapse collapse">
+                    <div id="collapseTwo" class="collapse" data-parent="#accordion">
                       <div class="card-body">
                         Anim pariatur cliche reprehenderit, enim eiusmod high life accusamus terry richardson ad squid.
                         3
@@ -1370,13 +1370,13 @@
                   </div>
                   <div class="card card-success">
                     <div class="card-header">
-                      <h4 class="card-title">
-                        <a data-toggle="collapse" data-parent="#accordion" href="#collapseThree">
+                      <h4 class="card-title w-100">
+                        <a class="d-block w-100" data-toggle="collapse" href="#collapseThree">
                           Collapsible Group Success
                         </a>
                       </h4>
                     </div>
-                    <div id="collapseThree" class="panel-collapse collapse">
+                    <div id="collapseThree" class="collapse" data-parent="#accordion">
                       <div class="card-body">
                         Anim pariatur cliche reprehenderit, enim eiusmod high life accusamus terry richardson ad squid.
                         3