Handle assertion of HTTP-redirect for a LogoutResponse in auth-saml2

The logout process was functional but Peertube was showing an 404 error while handling the LogoutResponse. Maybe Peertube v2 was simply redirecting to the root page instead of showing a 404 ?

This adds an endpoint to capture the HTTP-redirect. This also prevents the assertion of the saml LogoutResponse with post_assert as it is not an SamlResponse and will throw an error.