Use indexOf instead of includes because of old browsers.

src/components/join-flow/username-step.jsx

@@ -39,7 +39,7 @@ class UsernameStep extends React.Component {
     componentDidMount () {
         // Send info to analytics when we aren't on the standalone page.
         // If we are on the standalone join page, the page load will take care of this.
-        if (!window.location.pathname.includes('/join')) {
+        if (window.location.pathname.indexOf('/join') === -1) {
             if (this.props.sendAnalytics) {
                 this.props.sendAnalytics('join-username-modal');
             }