Make nbconvert html full output like notebook's html.

Basically makes the DOMs of both map one to one.  This doesn't mean string notebook html == string nbconvert output.

I used a custom in-house HTML diffing rig to diff, see here https://gist.github.com/jdfreder/9241376 (mentioned in the README.md inside nbconvert/tests).

related to #5084
closes #5224