Play buttons on long embedded notebooks

[doolittle]

The standard view of an embedded notebook is meant for short notebooks where the entire thing is visible in one screen height:

For extremely long notebooks, like our jsarrays this puts the "play" button out of site.

How should we fix this? With a floating play button at the top of the embed as well? Should we only display the top button when we know the notebook height is greater than the screenheight?

[blakeembrey]

@doolittle Progress on this is hindered by the fact that there is no native and fast cross-domain way to track the users viewport for scrolling.