Launch from HTTP referrer, autodetect launch URL

[manics]

Opening this for initial comments, not sure how much time I'll have to follow up in the immediate future.

Background

• https://discourse.jupyter.org/t/binderhub-button-pull-from-referrer/1572/
• This also implements some of the "awesome bar" suggestion from Awesome bar/landing page redesign #844

This adds a new /autodetect page that takes the HTTP referrer and converts it to a mybinder URL. You can also paste a GitHub repo URL, GitHub gist URL, GitLab repo URL, or Zenodo DOI into the input box. Currently this is a separate page, mostly copied from the main index.html.

Example: In a GitHub repo create a readme with a link to https://binder.example.org/autodetect, if it works the referrer will be parsed and converted into a link to launch the repo you came from.An earlier example can be seen at https://github.com/manics/mybinder-referrer/tree/demo

TODO:

• Tests
• Don't hard-code https://mybinder.org/

[meeseeksmachine]

This pull request has been mentioned on Jupyter Community Forum. There might be relevant details there:

https://discourse.jupyter.org/t/binderhub-button-pull-from-referrer/1572/9

[meeseeksmachine]

This pull request has been mentioned on Jupyter Community Forum. There might be relevant details there:

https://discourse.jupyter.org/t/tip-embed-custom-github-content-in-a-binder-link-with-nbgitpuller/922/41

[jd41]

I am making a note here hoping that it's remembered to update the README.md in JupyterLab once this PR is merged, see here for more information.

What's the procedure that this is not forgotten, where does such a comment properly belong?

[yuvipanda]

I've memorialized this feature in #1860 (since I didn't find anything else). Given that, I'm going to close this one.