UserManager stops resource loading if sign in fails

[semvankoutrik]

I noticed that on initial load of a React app all pending resources are cancelled if redirecting to sign in failed. This happens when using UserManager with the default RedirectNavigator, because inside its close-function returned by prepare() it stops the window (MDN: The window.stop() stops further resources loading). This causes some fonts and HTTP-requests to be cancelled which in turn causes a weird looking error message for the user.

I couldn't think of any particular reason to stop the window, but I thought I'd ask before creating a PR to be certain: is there a particular reason targetWindow.stop() is called? If there is, can it be made configurable to not close the window?

For now I'll just copy the navigator, remove the line and insert it in the UserManager.