- 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.
Until now we expected the .org file to print back to itself - we can't do that
when the input is not pretty printed already - with the introduction of blocks
with unindented content that will be the case.
