Handle footnotes in manual sections

[Trello ticket](https://trello.com/c/oiJYOedK/266-manuals-footnotes-frontend)

Manual sections containing footnotes now have an h2 added for the footnotes detail at the bottom of the body.

Style it appropriately.

When a user clicks on a footnote link, open the footnotes section so that the footnote content is visible.

(JavaScript with @alicebartlett)

This depends on https://github.com/alphagov/specialist-publisher/pull/217

Footnotes after this change in a manual (they weren't supported before this, so no point with an unstyled 'before')

![footnote](https://dl.dropboxusercontent.com/u/39836361/Screenshot%202014-08-11%2008.40.16.png)