fix: correct proptype for 'component' value in NavList

note: there seems to be no consensus on what the right proptype would be
facebook/react#5143
but after looking in node modules, it seems that react-router itself uses func
:shrug:

app/components/pages/StaticPages/index.js

@@ -77,7 +77,7 @@ StaticPage.propTypes = {
     PropTypes.shape({
       label: PropTypes.string.isRequired,
       link: PropTypes.string.isRequired,
-      component: PropTypes.object.isRequired,
+      component: PropTypes.func.isRequired,
     }),
   ).isRequired,
 }