prepend 'impactstory-' to several classes in embed.

This will keep them getting confused with other classes on the host
page. Not a perfect solution as there are a few other classes we apply
that could get confused, too, but much less likely than the ones
pointed out by Alf at PeerJ. Longer term fix, I think, is maybe getting
rid of boolean widget data- attributes, and making them all strings
for consistancy. This should work fine for now, though.