Set audience to the resource server by default

Change the way Access Tokens are issued by default, and set their audience to the Resource Server (instead of client_id). That way, the access_token in oidc-rp storage can actually be used as a simple way to communicate with a user's home pod.

As per https://github.com/solid/node-solid-server/issues/1061#issuecomment-459688994

Action items:

• Modify oidc-op's authentication and token requests to use the server's defaultRsUri as the default audience for the AccessToken. (PR https://github.com/solid/oidc-op/pull/16, in review)
• Modify oidc-auth-manager to pass in the serverUri, to serve as the default RS uri. (PR https://github.com/solid/oidc-auth-manager/pull/39, in review)
• Modify node-solid-server to use the updated oidc-op and oidc-auth-manager libs.