Fixed auth service login redirect query param to obey WebHostUrl settings

Occurs when running behind a load balancer - the `IHttpRequest.AbsoluteUri` contains the internal network port numbers, and these get printed straight into the redirect query string param.

Not 100% sold on my solution though - e.g. it currently strips any query string in the original url.

I'm thinking a more global solution is to actually modify `IHttpRequest.AbsoluteUri` property to obey the WebHostUrl setting and modify the hostname before returning. But that's a pretty central change so wanted to discuss it first.