Jupyter notebooks rendering

A Jupyter notebook contains markdown, metadata and code. But its file format is just JSON. It would be nice if the SHA were "notebook-aware" and showed them nicely.

The easy way is to show a link to nbviewer for every notebook file.

Just put the raw-file link and the nbviewer link together, like in

https://nbviewer.jupyter.org/urls/archive.softwareheritage.org/browse/content/sha1_git%3Ad55160e7c923017b137e9a4bbe43234596008bc3/raw//%3Ffilename%3Dconvert.ipynb```

The hard way is to convert notebooks on the fly to html (as github does).

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*Migrated from T1641 ([view on Phabricator](https://forge.softwareheritage.org/T1641))*