Support notebook 4.2

[bollwyvl]

Notebook 4.2 beta is out. Let's be ready to do a new release. Since it will be drastically better, I am proposing we this to a major rev, 3.0.0: we can continue to maintain the 2.x line if it makes sense, but likely there will be other reasons to move on... gotta love beta software!

This brings forward some earlier work, and adapts some new things:

• moves the nbextension install step to build.sh. enable is still in post-link.sh
  • this has conda handle the file movement in $prefix/share/jupyter
  • still contending on enable in $prefix/etc/jupyter
• removes install.py and dependencies (funcsigs, nb_config_manager)
• adds a post-link.sh (disables extensions... deletion is taken care of by conda)
• next steps:
  • [x] review by @damianavila?
• when actual 4.2 release drops: friday or monday, apparently
• when 4.2 on conda
  • [ ] fix deps
  • [ ] tag
  • [ ] merge
  • [ ] do CI build/upload
  • [ ] work with distribution team to update anaconda versions

[damianavila]

Minor comments... but LGTM... btw, I have uploaded a notebook 4.2b1 conda package into anaconda-nb-extensions... so if you re-add the channel, tests should pass... I think...

[damianavila]

Second view, LGTM...