Add reentrant conversion to Showdown footnotes.

Fixes #4668.

However, note that multi-paragraph footnotes are still broken. I'm not sure how to fix this; there seems to be the intention that it should work, but I don't really understand the code, and for my purposes, just inline formatting is good enough for my footnotes.
I don't actually know what gets generated, I was just going on a) what was there before, and b) that there's still a `</p>` below on line 53.  Does the generated markup have an opening `<p>` but not a closing one?