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.
Okay, I see what you're saying.  I pulled this down to test and it's still generating the same markup as before, so  that part seems fine.