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
Fix line width for headerless sections

When starting a document with something other than a <h2>, the body of
the section was being incorrectly displayed as full width. This commit
wraps everything in the body up until the first <h2> in a div allowing
us to style it.