Adding accounting config read/write to admin rest api and cli to set/get/rm them

This is similar to the perm and zone configs.

I'm going to try to consolidate some of the handler cli code next.

Let's also move this into a method which can be called from all three get handlers.

func marshalBody(req *http.Request, msg gogoproto.Message) ([]byte, error)

I realize this means we say goodbye to the ToYAML, ToJSON, ParseFromJSON, etc.  Good riddance. All of those variants were just duplicated code anyway.