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/web_hub_test.go

@@ -66,7 +66,7 @@ func TestHubStopWithMultipleConnections(t *testing.T) {
 	s := httptest.NewServer(dummyWebsocketHandler(t))
 	defer s.Close()
 
-	th.App.HubStart()
+	th.Server.HubStart()
 	wc1 := registerDummyWebConn(t, th.App, s.Listener.Addr(), th.BasicUser.Id)
 	wc2 := registerDummyWebConn(t, th.App, s.Listener.Addr(), th.BasicUser.Id)
 	wc3 := registerDummyWebConn(t, th.App, s.Listener.Addr(), th.BasicUser.Id)
@@ -84,12 +84,12 @@ func TestHubStopRaceCondition(t *testing.T) {
 	// So we just use this quick hack for the test.
 	s := httptest.NewServer(dummyWebsocketHandler(t))
 
-	th.App.HubStart()
+	th.Server.HubStart()
 	wc1 := registerDummyWebConn(t, th.App, s.Listener.Addr(), th.BasicUser.Id)
 	defer wc1.Close()
 
 	hub := th.App.Srv().hubs[0]
-	th.App.HubStop()
+	th.Server.HubStop()
 
 	done := make(chan bool)
 	go func() {
@@ -347,7 +347,7 @@ func TestHubIsRegistered(t *testing.T) {
 	s := httptest.NewServer(dummyWebsocketHandler(t))
 	defer s.Close()
 
-	th.App.HubStart()
+	th.Server.HubStart()
 	wc1 := registerDummyWebConn(t, th.App, s.Listener.Addr(), th.BasicUser.Id)
 	wc2 := registerDummyWebConn(t, th.App, s.Listener.Addr(), th.BasicUser.Id)
 	wc3 := registerDummyWebConn(t, th.App, s.Listener.Addr(), th.BasicUser.Id)
@@ -418,7 +418,7 @@ func BenchmarkGetHubForUserId(b *testing.B) {
 	th := Setup(b).InitBasic()
 	defer th.TearDown()
 
-	th.App.HubStart()
+	th.Server.HubStart()
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {