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

app/login.go

@@ -122,7 +122,17 @@ func DoLogin(w http.ResponseWriter, r *http.Request, user *model.User, deviceId
 		Secure:   secure,
 	}
 
+	userCookie := &http.Cookie{
+		Name:    model.SESSION_COOKIE_USER,
+		Value:   user.Id,
+		Path:    "/",
+		MaxAge:  maxAge,
+		Expires: expiresAt,
+		Secure:  secure,
+	}
+
 	http.SetCookie(w, sessionCookie)
+	http.SetCookie(w, userCookie)
 
 	return session, nil
 }