with a secret URL that can be generated from information in ~/.tahoe/private/control.key . General idea is to put node-local secrets and controls here: grid membership, checker/repairer management service controls, periodic local backup controls. This is like control.furl, but web-based.
still needs a lot of work
also update Account. Probably need to update tests too.
(don't enable remote callers to cancel starter leases by using "")