Don't show desktop notifications for messages entering the channel th… (#4216)

* Don't show desktop notifications for messages entering the channel the user is currently on

Coding style fixes

Allow notifications if browser tab is not focussed

* Don't show desktop notifications for messages entering the channel the user is currently on

Coding style fixes

Allow notifications if browser tab is not focussed

* Mute sound when a user doesn't get a desktop notification

* Also play sound on desktop notifications

* Fixed reviewer's remarks:
 * Removed double notification check
 * Removed direct links to browser store, created separate actions

webapp/actions/global_actions.jsx

@@ -535,3 +535,10 @@ export function toggleSideBarAction(visible) {
         });
     }
 }
+
+export function emitBrowserFocus(focus) {
+    AppDispatcher.handleViewAction({
+        type: ActionTypes.BROWSER_CHANGE_FOCUS,
+        focus
+    });
+}