Prevent parallel login flows after logout

[julien-nc]

Global problem: When the user is logged out (by user_oidc or anything else like integration_swp), user_oidc might try to redirect to the start of an Oidc login flow. This can lead to parallel login flows messing with each other (state mismatch) if all requests are redirected.

This can happen:

• after a logout, if allow_multiple_user_backends is set to 0 and there is only one provider, any request to /login will redirect to the user_oidc.login.login route
• if store_login_token is enabled and a token expires, we reach TokenService::reauthenticate which logs out and redirects to the store_login_token

Solution in this PR: Only redirect to the login flow when the request comes from a 'navigation' context.

This is an alternative to #1409 which is possibly better.

[CarlSchwan]

Suggested change

	if ($request->getHeader('X-Requested-With') === 'XMLHttpRequest') {
	return false;
	}
	return true;
	return $request->getHeader('X-Requested-With') !== 'XMLHttpRequest');