This opens a wormhole and sends appropriate JSON down
it to a tahoe-gui using a wormhole server running on
tahoe-lafs.org
The other end uses the 'tahoe create-node' command (with
new --join option) to read the configuration JSON from
a 'tahoe invite' command
Squashed all commits that were meejah's between
30d68fb499f300a393fa0ced5980229f4bb6efda
and
33c268ed3a8c63a809f4403e307ecc13d848b1ab
On the branch meejah:1382.markberger-rewrite-rebase.6 as
per review