Remove all remaining occurences of fakeApp (#17661)

* Remove all remaining occurences of fakeApp

Migrated any remaining methods under server.

And for everything else, renamed fakeApp to app.
The word fakeApp is confusing and we should just call
it for what it is - an app.

https://focalboard-community.octo.mattermost.com/workspace/zyoahc9uapdn3xdptac6jb69ic?id=285b80a3-257d-41f6-8cf4-ed80ca9d92e5&v=495cdb4d-c13a-4992-8eb9-80cfee2819a4&c=639a0bc1-4401-43d5-81ec-0dd54e796d9a

```release-note
NONE
```

* fix tests

app/notification_push.go

@@ -262,13 +262,9 @@ func (a *App) UpdateMobileAppBadge(userID string) {
 
 func (s *Server) createPushNotificationsHub() {
 	buffer := *s.Config().EmailSettings.PushNotificationBuffer
-	// XXX: This can be _almost_ removed except that there is a dependency with
-	// a.ClearSessionCacheForUser(session.UserId) which invalidates caches,
-	// which then takes to web_hub code. It's a bit complicated, so leaving as is for now.
-	fakeApp := New(ServerConnector(s))
 	hub := PushNotificationsHub{
 		notificationsChan: make(chan PushNotification, buffer),
-		app:               fakeApp,
+		app:               New(ServerConnector(s)),
 		wg:                new(sync.WaitGroup),
 		semaWg:            new(sync.WaitGroup),
 		sema:              make(chan struct{}, runtime.NumCPU()*8), // numCPU * 8 is a good amount of concurrency.