Refactor: Move /rsvp/submit into /app

Part 1 in putting the Node.js code in one spot.
Refactor: Move /data into /app

Part 2 in moving all the Node.js code into one
location.
Refactor: Remove old client.js
Refactor: Rename /app to /_app

Make it more obvious that 'app' is hidden by putting
an underscore in front of it.
Refactor: Move /lib/secrets.js to /secrets.js

For easier deployment, move the secrets config file
out of the lib folder.