Remove app initialization from WebHub (#18687)

- Make it a Server method.
- Pass Server instead of App.
- Remove global app instance from NewServer, rather
create local instances whenever needed.
- Remove App from Websocket router, and create dynamically
on every request.
- Remove HubStart and HubStop from App methods.
- Explicitly using s.Log instead of the global logger
to indicate dependency on Server. We could have passed the logger
explicitly but it doesn't look ideal.

```release-note
NONE
```

app/webhub_fuzz.go

@@ -153,7 +153,7 @@ func Fuzz(data []byte) int {
 
 		s := httptest.NewServer(dummyWebsocketHandler())
 
-		th.App.HubStart()
+		th.Server.HubStart()
 
 		u1 := th.CreateUser()
 		u2 := th.CreateUser()