Break stats out of the 768px box

Let's ditch the 768px max width on the stats view to free up more space on larger screens. This will also put us in a position to use some of the cooler stat displays from Calypso. CC @keoshi since it might effect the apps ad.

I believe Calypso implemented this in a way that makes that narrow size the default and an additional prop adds a class to override the max width for the wider view.