Composable API handler, support v1 and v2 login.

Some refactoring & restructuring of srvRoot to support the existing Admin.Login API as well as allowing us to introduce a v2 Login API, to be proposed in a followup pull request.
I'd probably name this "authedAPI" if we want that syntax, since this is the API after you have authenticated. "authAPI" sounds *to me* like it is the API you use *to do auth*.