Switch to using IdentityServer4 demo instance

[jeroenheijmans]

This repo currently relies on the IdentityServer3 instance graciously provided by the maintainer of angular-oauth2-oidc. However, there is also a demo instance of IdentityServer4, provided by the IDS4 authors. I think it's preferable to try and use that instance by default, integrating with the latest version of IdentityServer.