[MM 7970] Maintain online status while the Desktop App is in the background ... and other things. (#993)

* monitor os-level user activity

* disable eslint warnings

* add tests for UserActivityMonitor

* couple more tests

* udpate headers of new files

src/browser/js/WebappConnector.js

@@ -0,0 +1,17 @@
+// Copyright (c) 2016-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import EventEmitter from 'events';
+
+/**
+* Provides the bridging infrastructure to connect the desktop and webapp together
+*/
+export default class WebappConnector extends EventEmitter {
+  constructor() {
+    super();
+
+    this.active = true;
+
+    window.webappConnector = this;
+  }
+}