[Delivers #82528174] Playlab Embeddable iFrame

This feature allows any iFrame to be embedded into e.g. a multiple-choice assessment question.
It also creates a stripped-down embeddable format suitable for embedding Play Lab games into a small frame:
![image](https://cloud.githubusercontent.com/assets/56541/5022316/e121f948-6a97-11e4-854f-969189dd347e.png)


@laurelfan I considered creating a separate 'embedded' layout, but decided against it since we were already parameterizing the existing application layout for other view contexts (e.g. `no_padding`, and removing the footer for mobile). Also the existing layout is not very complex (less than 40 lines) and a new layout would duplicate much of this structure (we would still need to include the javascripts/stylesheets, base HTML tags, analytics, etc. There is a tradeoff though and I agree that if the different layout types become any more complex we should probably separate them out.