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.

Content-types can have additional parameters (e.g. `Content-Type: application/json; charset=utf-8`); we should strip those off before switching on the base type. (And for bonus points reject any request which specifies a charset other than utf8, although that's probably not worth doing unless we start using a more full-featured web framework where we can abstract this out. Just break at the first semicolon)