dropdown_list_widget: Bind Escape key handler to close dropdown.

In commit 0d20a20, we missed out the "Escape" keyhandler
which is required to handle scenario where a user wants
to close the dropdown using "Escape" key.

This commit essentially handles the above case, which
enables the functionality to close dropdown using
"Esc" keyboard button.

static/js/dropdown_list_widget.js

@@ -154,6 +154,10 @@ export class DropdownListWidget {
                     }
                     break;
                 }
+                case "Escape": {
+                    $dropdown_menu.dropdown("toggle");
+                    break;
+                }
             }
         });
     }