Do not send push notifications for channels being actively viewed (#3931)

webapp/components/needs_team.jsx

@@ -94,6 +94,7 @@ export default class NeedsTeam extends React.Component {
 
         $(window).on('blur', () => {
             window.isActive = false;
+            AsyncClient.setActiveChannel('');
         });
 
         Utils.applyTheme(this.state.theme);