Get the choice screen to work as well

Bonus architecture fixes during this round:
  - Support the ability for the result screen of a client to embed the result screen of other clients
  - Stop reusing the same "score" field for all clients, and move to each client managing his/her set of custom fields. This was particularly important for choice, since it had several results, but also provided a cleaner interface for the other clients.
 - Started switching "default" functionality to a separate client called "default"
 - Added support for callbacks from javascript to track which events are triggered. This can be reused to measure engagement with the result screen.