Handle footnotes in manual sections

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)
Merge pull request #63 from alphagov/handle-footnotes-in-manual-sections

Handle footnotes in manual sections