Convert footnotes back to footnotes.

I'd originally written these posts on Tumblr using their footnote
syntax which is PHP Markdown Extra or something like it; after
rendering by Tumblr and then importing here with Jekyll's tumblr.rb
migrator script which runs it through nokogiri and html2text instead
of obtaining the original Markdown source, the footnotes had been
baked (and the rendering of the footnote bodies was reasonable but the
links had been destroyed in the baking).

Here I manually converted them back to PHP Markdown Extra footnote syntax.