Create minimal sinatra dashboard app

Based on https://github.com/edds/display-screen
Stub javascript that fetches from realtime api

Again based on https://github.com/edds/display-screen

* Create an app, app manager and app realtime
* Load js app on index page
Retrieve realtime user information from GA
Create rolling slides

Based on https://github.com/Demotive/gds-data-screens
Create users online slide

Base styles on
https://github.com/Demotive/gds-data-screens