Add back to contents link to html pubs

Stuck the link to the bottom of the page as we decided that it was less
visible and less ocnfusing as to what was happening when it disapeard
behind a heading number.

Slightly tweeked the large numbers so that they have background to hide
the jump link as it scrolls past.

Updated the JS to handle sticky things at the bottom of the page as well
as the top.

https://www.pivotaltracker.com/story/show/46442875
Update copy on history page for blogs

https://www.pivotaltracker.com/story/show/48801463
Remove extra indent on history page
Unify styling of headers with lead-in paragraph

There were 4 sets of styles for effectivly the same block. Unified those
styles to use the same helper and reduced the markup by getting rid of
legacy code for more complicated styling.
Merge pull request #509 from alphagov/history-update

Update history text and extra styling

https://www.pivotaltracker.com/story/show/48801463
Merge pull request #508 from alphagov/html-pub-skip-link

Add back to contents link to html pubs