RFC 503: Create smart sub repo permissions client

Created by: ryanslade

The client is responsible for checking whether a given user can access a specific piece of content inside of a repo.

Details here: https://docs.google.com/document/d/1d8j-6VC_nk8HXEDT6U2_s-_9uSzgzHWZzrJjII9pKEE/edit#heading=h.4w9htz1j4hqp