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/utils/constants.jsx

@@ -156,7 +156,9 @@ export const ActionTypes = keyMirror({
     SUGGESTION_CLEAR_SUGGESTIONS: null,
     SUGGESTION_COMPLETE_WORD: null,
     SUGGESTION_SELECT_NEXT: null,
-    SUGGESTION_SELECT_PREVIOUS: null
+    SUGGESTION_SELECT_PREVIOUS: null,
+
+    BROWSER_CHANGE_FOCUS: null
 });
 
 export const WebrtcActionTypes = keyMirror({