Add aria-labels to footnotes and footnote backlinks

[stevenjmesser]

Related to #692, here's a pull request to make footnotes accessible to screen readers.

[gettalong]

You are using rel="footnote" (lowercase) but in some tests "Footnote" is written. So this would need to be fixed.

Furthermore, I don't want to introduce an English language default which would mix languages when the document is not written in English. Is aria-label meant to be an "internal" reference like with the role attribute? Or is this something a screen reader would actual read out loud?

If it is the latter, we would need to introduce a new option like footnote_labels that defines the text of the labels, with the default being off.

[kinoute]

any update on this?

[stevenjmesser]

I need to think about how to address @gettalong's comments as I'm not a developer, @kinoute! But do you have any ideas? I think I'd need to think about multilingual support most...

[stevenjmesser]

Closing as addressed in #773