Provide access to the read-only parts of the Great Black Swamp interface

After #141 (closed) is fixed we will have a Tahoe-LAFS that can speak Great Black Swamp. A read-only client can do useful stuff with this, even though we don't have ZKAPAuthorizer integration for GBS yet.

We need to turn on GBS and allow access to it to authorized clients. If we allow read-only actions then we're not giving out anything more than clients can already get from the Foolscap interface. If we allow write actions right now then clients will be able to do unauthorized storage allocating operations so try to avoid that.