Refactor preview JS

These changes allow the preview button to be placed in a different
position in the page from the preview container.
Fix for preview not working on new spec docs
Redesign of manuals #index and #show

As part of the ongoing development, I've redesigned the #index and #show
pages to better match the designs. Some workflows have changed, but this
will be an ongoing redesign.
Redesign of manual document show
Merge pull request #90 from alphagov/refactor-manual-views

Refactor manual views and preview code