Merge asides with footnotes

[agoose77]

Proposal

In https://hackmd.io/Q_to9V1cS-2Q0RGEFFIm7w, we discussed the fact that asides and footnotes are very similar. In https://edwardtufte.github.io/tufte-css/ they're only distinguished by styling (placement).

At the AST level, we could merge these.