Add ReviewNB for to compare notebooks (ipynb)

[wildintellect]

I attempted to add https://github.com/apps/review-notebook-app/installations/new so we can diff notebooks on PRs but it doesn't seem to show up yet in PRs. Not sure if there's an organization setting @jedsundwall that needs to allow the application.

[wildintellect]

It's working now on any PRs with ipynb, just needed some patience.

[jedsundwall]

Yeah sorry I should have told you I approved your request. Not sure if you got a notification about that.