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