Extends the shipped CFG_MASTER_DIRECTORY_URL/JOIN_KEY join flow with the
selfUrl a spoke needs to register itself for live catalog replication
(theta-directory v2.4.0's POST /api/site/spokes) -- without this, every
spoke was permanently limited to the one-time join snapshot even after
the master gained the ability to push live updates.
setup.sh already computes CFG_SSO_HOST before this point in the script;
passes https://$CFG_SSO_HOST as bootstrap/site-join.js's third argument,
which forwards it as `selfUrl` in the POST /api/site/join body.
wmantly
f42b69c084