So, I'd guess the semantics of `.../registrations/current` are:
 - get the current user's id
 - look up a registration with this id, and the given conference
Right?

I wonder if it would be better to have the client just know the user's id, and request the registration by id.  Maybe it'd end up more work if `registrations` is the only endpoint that offers a 'current' subpath, but maybe not.  I just have a gut feeling it'd be better to separate it out.  

So I guess keep this for now, but we may want to rethink this decision later.