[148.4] Configure App to publish JukeboxCreated PubSub events

[peddybeats]

• Create pubsub_client.js
• Expose init()
  • Should instantiate the pubsub client
• Expose publish(topic_id, message)
  • Should use global pubsub client and get a publisher isntance
  • Should return a promise that lets the caller consume result
• In server.main() call pubsub_client.init()
• In app.generateJukebox(), at the end of the promise chain, call pubsub_client.publish(JUKEBOX_CREATED_TOPIC_ID, jukeboxId)