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.
That means the code isn't clear, which I should have realized; I was mostly in a hurry to get my blog footnotes working.

The trick is that the generated Markdown will have its own P tag. However, this is an assumption that I'm not sure always holds.