Problem: inactive cell not flat and flushed
Problem: inactive cell not flat and flushed

Solution: Only darken the execution count section on hover, active, focus

We really need the concept of an active cell, which we're still aiming for.
Wanted to get closer to the mock for people to see.

Problems:
* cell is only "active" according to the mouse (since we can't style
  relatively with the current hierarchy)
* style of the inactive cell is hardcoded to match our current codemirror theme

Those two need to be solved separately: active cells and theming

rgbkrk/nteract,nteract/composition,nteract/nteract,rgbkrk/nteract,jdfreder/nteract,jdetle/nteract,nteract/nteract,nteract/composition,nteract/nteract,nteract/nteract,captainsafia/nteract,captainsafia/nteract,jdfreder/nteract,jdetle/nteract,jdetle/nteract,jdfreder/nteract,temogen/nteract,jdetle/nteract,temogen/nteract,0u812/nteract,rgbkrk/nteract,0u812/nteract,nteract/composition,captainsafia/nteract,temogen/nteract,nteract/nteract,jdfreder/nteract,rgbkrk/nteract,0u812/nteract,rgbkrk/nteract,captainsafia/nteract,0u812/nteract