The previous `UnreferencedFootnotesDetector` transform was untested and missed warnings for a number of cases.
This commit adds a test, to cover a reasonable range of scenarios,
then changes how the detection works to pass this test.
The transform now runs just after the docutils `Footnote` resolution transform
(changing its priority from 200 to 622)
then simply check for any footnotes without "back-references".
- Remove now-removed configuration values
- Use obvious 'unset' defaults for ``project`` and ``author``
- Prefer 'e.g.' to 'ex.'
- Allow non-list sequence types in various configuration values
- Add types and defaults to every confval directive
