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
We have [defaultState](https://developer.zendesk.com/apps/docs/agent/templates#defaultstate) property to do this for us.