WIP: Modifying iframe_sample_app to send and receive messages

:construction: Earlier the iframe_sample_app used to display content from an external website (Wikipedia).

/cc @zendesk/quokka 

### References
- Issue #59

### Risks
 - None
It was here: https://github.com/zendesk/demo_apps/blob/master/external_api_sample_app/app.js#L208
But I think it is passing the complete form data, like you said. So I'll simplify it here