Correct use of Node type in EventBoundary

This is a small fix for commit dab0a73 which resolves the Flow error
concerning the use of the type `Node` in the props and return value of the
EventBoundary component

Author: rlamacraft

src/main/webapp/ui/src/components/EventBoundary.js

@@ -1,14 +1,14 @@
 //@flow
 
-import React from "react";
+import React, { type Node } from "react";
 
 /**
  * All of the DOM events that happen inside of a dialog shouldn't propagate
  * outside as the menu will take them to be events that it should respond to by
  * providing keyboard navigation. See ../../../../QuirksOfMaterialUi.md, secion
  * "Dialogs inside Menus", for more information.
  */
-export default ({ children }: { children: Node }) => (
+export default ({ children }: { children: Node }): Node => (
   /*
    * The eslint suppression is required because `div`s should not ordinarilly
    * have event handlers as they cannot be focussed. However, in this case, we