Fixes #12171: Ensure panel groups have a bottom margin since we nuke it on child panels

dist/css/bootstrap.css

@@ -4881,6 +4881,9 @@ a.list-group-item-danger.active:focus {
   border-bottom-right-radius: 3px;
   border-bottom-left-radius: 3px;
 }
+.panel-group {
+  margin-bottom: 20px;
+}
 .panel-group .panel {
   margin-bottom: 0;
   overflow: hidden;

less/panels.less

@@ -150,6 +150,8 @@
 // the help of our collapse JavaScript plugin.
 
 .panel-group {
+  margin-bottom: @line-height-computed;
+
   // Tighten up margin so it's only between panels
   .panel {
     margin-bottom: 0;