MM-26575: Move app initialization inside Websocket router out of handler (#14991)

* MM-26575: Move app initialization inside Websocket router out of handler

The websocket router struct is shared amongst multiple handlers. Therefore,
there is a race condition while setting the app field and reading from it.

We move the initialization of the app field when the router struct is initialized.

And we also remove the initializations of some app fields which caused race
conditions by being written from multiple goroutines. They are already being
written once inside the AppInitializedOnce.Do method and it's redundant
to set them again to the same value.

* Add missing fields in server connector

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/server.go

@@ -392,6 +392,7 @@ func NewServer(options ...Option) (*Server, error) {
 		server:   s,
 		handlers: make(map[string]webSocketHandler),
 	}
+	s.WebSocketRouter.app = fakeApp
 
 	if appErr := mailservice.TestConnection(s.Config()); appErr != nil {
 		mlog.Error("Mail server connection test is failed: " + appErr.Message)

app/websocket_router.go

@@ -26,7 +26,6 @@ func (wr *WebSocketRouter) Handle(action string, handler webSocketHandler) {
 }
 
 func (wr *WebSocketRouter) ServeWebSocket(conn *WebConn, r *model.WebSocketRequest) {
-	wr.app = New(ServerConnector(wr.server))
 	wr.app.InitServer()
 
 	if r.Action == "" {