web: add "private control panel"

This uses 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.

Also add a mockup of invitation generator/acceptor pages.
start adding InvitationManager
add field for private notes, try to improve layout a bit