put short slug in Simple Footnotes

put short slug in Simple Footnotes

Currently, "Does General Intelligence ...?" and "Reply to Ozymandias
on Fully Consensual Gender" are both on the front page, and the
earlier-on-the-page footnote fragments clobber later ones of the same
number. Including part of the post slug in the fragment solves the
problem, and I'm going all the way to five segments because you could
imagine two "Reply to Author on Topic" posts being on the same page
(even though I'm likely to abandon this plugin (for future posts, or
altogether) because native-Markdown footnotes (those are a thing!!)
are friendlier to read and write than the `[ref]` notation.