Migrate pelican-jupyter to liquid-tags

[warrenalphonso]

Looks like pelican-jupyter isn't maintained anymore and it's on an older version of nbconvert

[warrenalphonso]

Won't fix.

It'd be neat to use a plugin to embed Jupyter Notebooks in blog posts. The main
problem is that making the Notebook CSS play well with our site CSS wasn't straightforward,
and while I could use some hacks
I didn't want to depend on a static nbconvert version (nbconvert chooses
the CSS version).
I can revisit this if there's more documentation and stability around Jupyter
Notebook CSS classes.

Alternatively, I could convert to Markdown and then style the input/output cells
myself. I didn't want do that.