Refactor footnote parsing to use DOM parsing rather than RegEx

[Rumperuu]

This is going all the way back to what originally brought me into the project: the use of RegEx to parse posts for footnotes is prone to errors, and in order to respond to each of these the RegEx in task.php has grown into a bit of a beast.

A better, more reliable solution, as I mentioned at the time, would be to properly parse the DOM instead.

[markcheret]

There's also an idea to discuss regarding saving footnotes in DB:
https://wordpress.org/support/topic/possible-to-reuse-footnotes/ (see original poster and last comment)
Or have footnotes also survive in the DOM if the plugin is deleted... just an idea.

[Rumperuu]

https://stackoverflow.com/questions/1732348/regex-match-open-tags-except-xhtml-self-contained-tags/1732454#1732454