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.
Not sure if I'm understanding the code correctly, but `<p>` is redundant inside a `<li`> tag and is not required