don't split `.cell` and `div.cell` CSS

`.cell` matches non-IPython things that it shouldn't (inside highlighted code),
everything `.cell` should match in IPython is a div.
fab css
Merge pull request #4159 from minrk/minor-style

don't split `.cell` and `div.cell` CSS

I know we have bigger CSS scoping issues to deal with, but this is an easy fix for an obvious problem.