updates header role and modernBrowser check in app initializer

Author: george

app/index.js

@@ -1,8 +1,7 @@
 import "core-js/features/promise"
 
 const polyfills = []
-const modernBrowser =
-  "assign" in Object && typeof Promise === "function" && typeof Symbol === "function"
+const modernBrowser = "assign" in Object && "from" in Array && "fetch" in window
 
 if (!modernBrowser) {
   polyfills.push(import(/* webpackChunkName: "polyfill" */ "core-js/stable"))

app/layouts/Main/Main.js

@@ -65,7 +65,7 @@ export default class Main extends React.Component {
           tabIndex={headerTabIndex}
           ref={this.headerRef}
           onBlur={this.handleHeaderBlur}
-          role="heading"
+          role="banner"
         >
           <GlobalNav handleMainFocusClick={this.handleMainFocusClick} />
         </header>

app/layouts/Main/__tests__/__snapshots__/Main.spec.js.snap

@@ -27,7 +27,7 @@ exports[`<Main /> #render renders 1`] = `
     <Main>
       <header
         onBlur={[Function]}
-        role="heading"
+        role="banner"
         tabIndex={null}
       >
         <GlobalNav