- Remove unused footnote section title option
- Move away from maintaining a list of footnotes in the document (only needed
for html export, potential maintainance overhead when modifying the document)
and rather only build it on export when required.
- HTML export: Rename all footnotes to numbers (so we can support anonymous
footnote references by assigning them a number) and export footnotes in order
of reference, not definition. The implementation of this makes it natural to
also stop exporting unused footnote definitions so we do that as well.
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