Fix govspeak CTA styling

The CTA box was indented to the left, a throwback to the days when
lists were indented this way. This lines the CTA box up with the main
flow of content and also makes sure ordered lists are correctly
aligned.
Make HTML pub CTA consistent with other content

HTML publication CTAs now use the same grey box as normal documents.
This also fixes an issue with the rendering of lists within CTAs.
Merge pull request #813 from alphagov/56344734-cta-box-fixes

56344734 Fix CTA box styling

https://www.pivotaltracker.com/story/show/56344734