--mathjax does not work consistently

[nomeata]

In Debian, the files we ship must not rely on external data, so we cannot use mathjax from the CDN. Luckily, haddock provides a --mathjax flag that allows to specify the URL to use, which we do (--mathjax=file:///usr/share/javascript/mathjax/MathJax.js).

Unfortunately, it does not work everywhere: It has an effect on index.html, but not other HTML pages generated by haddock.

The problem is that in haddock-api/src/Haddock/Backends/Xhtml.hs, maybe_mathjax_url is not passed around consistently, and sometimes headHtml is then called with Nothing.

It would probably be (type-)safer to pass this as a String and not as a Maybe String, and set the default in the argument parser. This way, you cannot “forget” to pass it.