Update old vendored CSS and JS to more modern, supported and secure versions #48
Description
Right now, the vendored CSS includes 7 year old versions of Bootstrap (3.3.5) and pygments from 2015; the latter at least should be a mostly drop-in upgrade and we can at least bump the former to 3.4.1, if not v4 or v5.
More concerning, the vendored JS dates from the same vintage, including the corresponding Bootstrap 3.3.5, JQuery 1.11.3 and an old copy of Python.org's "copy button" script (which I'm not sure even works now, as I don't see the button and it explicitly notes it "only works with JQuery 1.8.3 or less".
Also, mathjax is apparently attempting to be fetched from cdn.mathjax.org, which is shut down and generates a console warning (I guess its not vendored locally?). Not sure what's going on there.
Finally, the HTML template still has a XHTML 4 Transitional doctype; looks pretty straightforward to bump that to HTML 5 (I didn't notice any other particular problems there).
@SylvainCorlay , do you think any of your developers could take a look at helping pay off this techdebt? Its not ultra-urgent but Its going to be a problem sooner or later, and may particularly concern security-conscious corporate deployments.