Backport PR
#4159: don't split `.cell` and `div.cell` CSS...
Backport PR
#4159: 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.
I know we have bigger CSS scoping issues to deal with, but this is an easy fix for an obvious problem.
backport for 1.1