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
Mmm, do you remember which one by any chance? I'd would personally only do this if I was passing the whole form to be processed by some other code. We should have as little code as possible in these apps, though I know some demo apps are fairly complicated.