Fix view logic for HMRC leaf nodes

As HMRC leaf nodes are now on the document.body, make sure they aren't accidentally made collapsible.
I'm not sure if it's possible for the HMRC API to give us a document with a `document.body` AND `document.section_groups` but it seems sensible to show them both if they're both there.

Non HMRC manuals should just have a `document.body` on their show action, but they don't need a don't need to be wrapped in a `.body-content-wrapper` class because their headings still need to be full width (as visually they are styled differently to hrmc's manuals)

Further work on this file (which I'm leaving here as a note for myself later) would be to remove:
`
<div class='title-controls-wrap'></div>
<div class='collapsible-subsections'>
`
and leave add the javascript to add in these elements as they are not needed unless the javascript is running.
Add width restricting CSS for body content

HMRC content and any body copy under a subsection now has this class to restrict it's width to two thirds on body copy
Add .body-content-wrapper class

This commit adds .body-content-wrapper to the javascript body copy, removing the body copy's width as a concern for the .js-subsection-body class.

This means HMRC manuals and regular manuals can share this class to share their width (and possibly other styles)
Merge pull request #116 from alphagov/fix-collapsibleness

Fix collapsibleness