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.

I think this check is probably unnecessary (the Unmarshal functions should reject anything that's not utf8), but if we do need it it should be moved down into the FromJSON/FromYAML functions. We could accept binary-encoded protobufs too, and those wouldn't be valid utf8.