PLT-6492 Use new cookie to determine if user is logged in (#6317)

* Use new cookie to determine if user is logged in

* Add temporary code for 3.9 to prevent forced re-login

model/session.go

@@ -11,6 +11,7 @@ import (
 
 const (
 	SESSION_COOKIE_TOKEN  = "MMAUTHTOKEN"
+	SESSION_COOKIE_USER   = "MMUSERID"
 	SESSION_CACHE_SIZE    = 35000
 	SESSION_PROP_PLATFORM = "platform"
 	SESSION_PROP_OS       = "os"