Add aria-haspopup and aria-expanded attributes to dropdown plugin

1. Add aria-haspopup="true" and aria-expanded="false" to dropdown docs
2. Toggle aria-expanded between "true" and "false" when opening and closing dropdown

Satisfy item 1 from https://github.com/paypal/bootstrap-accessibility-plugin#dropdown. See #13553.

js/dropdown.js

@@ -42,7 +42,9 @@
 
       if (e.isDefaultPrevented()) return
 
-      $this.trigger('focus')
+      $this
+        .trigger('focus')
+        .attr('aria-expanded', 'true')
 
       $parent
         .toggleClass('open')
@@ -88,11 +90,13 @@
     if (e && e.which === 3) return
     $(backdrop).remove()
     $(toggle).each(function () {
-      var $parent = getParent($(this))
+      var $this = $(this)
+      var $parent = getParent($this)
       var relatedTarget = { relatedTarget: this }
       if (!$parent.hasClass('open')) return
       $parent.trigger(e = $.Event('hide.bs.dropdown', relatedTarget))
       if (e.isDefaultPrevented()) return
+      $this.attr('aria-expanded', 'false')
       $parent.removeClass('open').trigger('hidden.bs.dropdown', relatedTarget)
     })
   }