Refactor footnote handling

- 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.

org/testdata/misc.html

@@ -409,7 +409,7 @@ src/example/export blocks should not be converted!
 <a href="https://github.com/chaseadamsio/goorgeous/issues/87">#87</a>: Markup in footnotes is rendered literally
 </h3>
 <p>
-footnotes can contain <strong>markup</strong> - and other elements and stuff <sup class="footnote-reference"><a id="footnote-reference-2" href="#footnote-2">2</a></sup>
+footnotes can contain <strong>markup</strong> - and other elements and stuff <sup class="footnote-reference"><a id="footnote-reference-0" href="#footnote-0">0</a></sup> <sup class="footnote-reference"><a id="footnote-reference-1" href="#footnote-1">1</a></sup>
 </p>
 <h3 id="headline-23">
 <span class="todo">DONE</span>
@@ -472,7 +472,7 @@ Footnotes
 <hr class="footnotes-separatator">
 <div class="footnote-definitions">
 <div class="footnote-definition">
-<sup id="footnote-1"><a href="#footnote-reference-1">1</a></sup>
+<sup id="footnote-0"><a href="#footnote-reference-0">0</a></sup>
 <div class="footnote-body">
 <p>
 a footnote <em>with</em> <strong>markup</strong>
@@ -492,7 +492,7 @@ because that&#39;s possible
 </div>
 </div>
 <div class="footnote-definition">
-<sup id="footnote-2"><a href="#footnote-reference-2">2</a></sup>
+<sup id="footnote-1"><a href="#footnote-reference-1">1</a></sup>
 <div class="footnote-body">
 <p>
 that also goes for <strong>inline</strong> footnote <em>definitions</em>