Automatically convert spaces to non-breaking spaces where necessary around punctuation

This is about themeing / JavaScript improvements to the content.

In French, we use [non-breaking spaces](https://en.wikipedia.org/wiki/Non-breaking_space) before and after some punctuation elements, notably:
- Unbreakable space before `? ! ; : »`.
- Unbreakable space after `«`.

Wikipedia says it should be a "narrow non-breaking space" ` `, in 2000 when I was writing websites I used ` `, not sure what's the standard but either would do - the most important is that these punctuation signs don't end up separated from the preceding word (or following word for `«`) when the line wraps. I.e. avoid this:

I think other languages might also have similar requirements.

Asking translations to use those is a bit difficult, as there's no easy way to input them on most keyboards, and they're not easy to distinguish from a plain ascii space either.

So I suggest doing a search and replace as a post-processing step in JavaScript. Can something like this be done? CC @Calinou @pycbouh

We might also want to extend this to other typographical improvements, such as replacing `...` with `…`, but that might warrant discussion and properly listing all cases we want to handle. I'd suggest starting with the nbsp's as they're quite important.