[MRG] Check if nbviewer URL would show an error

[betatim]

If the nbviewer widget would show a 404 or other error we don't want to
display it. We check the status before rendering the template by making
a HEAD request to nbviewer, if the status code is >=400 we don't show
the preview.

closes #922

[betatim]

@lesteve I only saw your comment after making this PR. Sorry :-/

This implements your idea using the tornado HTTP client instead of urllib. The code has to work async/non-blocking and I don't think urllib supports that (yet).

It isn't quite the feature requested in #816 because it doesn't give the choice of disabling or not to the user. However this is a quick win, we can still add the "disable even if the preview works" options if we decide we want that.

[lesteve]

Very nice to see this, thanks a lot!

No worries about superseding my naive attempt. I got binder setup locally using minikube and learned a few useful things along the way!

[lesteve]

Very nice to see this merged! Naive question: I assume I have to monitor the commits in https://github.com/jupyterhub/mybinder.org-deploy in particular the changes in values.yaml (I have read the blog post https://blog.jupyter.org/automating-mybinder-org-dependency-upgrades-in-10-steps-bb5e38542059).

Are there additional steps I need to be aware of until this change reach mybinder.org?

Note: I am not in a hurry at all, I am asking out of curiosity and to have some very rough time estimate when I can give this a whirl on mybinder.org.

[betatim]

We have a bot (henchbot, just referenced this PR) that opens a PR on the deploy repo. The bot runs about once an hour and usually(depends on where people are, if it is night, weekend, etc) someone clicks merge an hour or two after the bot opened the deploy PR. Then it is live!

[lesteve]

OK that matches what I understood from the blog post, thanks for confirming this!