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);

webapp/components/post_view/post_view_cache.jsx

@@ -4,6 +4,7 @@
 import PostViewController from './post_view_controller.jsx';
 
 import ChannelStore from 'stores/channel_store.jsx';
+import * as AsyncClient from 'utils/async_client.jsx';
 
 import React from 'react';
 
@@ -28,6 +29,7 @@ export default class PostViewCache extends React.Component {
     }
 
     componentWillUnmount() {
+        AsyncClient.setActiveChannel('');
         ChannelStore.removeChangeListener(this.onChannelChange);
     }