"reinstate" links

[pkra]

For #13 (in particular #13 (comment)), we stopped "ignoring" links, thus adding role=presentation (as MathJax and SRE don't treat links as first class citizens).

However, this means we disable links even if a tree walker is unavailable. Instead, following the example of tabindex, we should leave anchors with href alone and let tree walkers disable them. That way, links will remain functional if the walker doesn't load.

[pkra]

Since MathJax/SRE won't have anchors in the SRE tree [output], we don't have to worry about the old situation where we worried about setting role=treeitem. Instead, we need to avoid setting role=presentation.