release(v2.4.0): live replication, coordinated promotion, mesh UI

Rolls up this pass's multi-site work: live catalog replication (spokes
stay synced after joining, not just a one-time snapshot), identical
agent-signing keys across sites, coordinated master promotion with real
old-master demotion, the UI to actually see and use any of it, and two
real bugs found only by live two-container testing (site-promote's dead
authorization check, and masterJoinKey/replicationPushToken leaking to
the browser via GET /api/site/config).

See CHANGELOG.md for the full list.

nodejs/package.json

@@ -1,6 +1,6 @@
 {
   "name": "t42-theta-directory",
-  "version": "2.3.0",
+  "version": "2.4.0",
   "description": "A very simple LDAP management and SSO system",
   "author": [
     {