Fix margins for different device widths

Using the .inner class to remain inline with other html on front end.
Fix some alignment issues with ie6

Some combination of the article tag and div.inner was causing content inside and outside of the article tag to render very far to the left in ie6.
Also switched section tag for main in line with other views on frontend.
Merge branch 'travel-advice-index' of https://github.com/alphagov/frontend into travel-advice-index
Merge branch 'travel-advice-index' of https://github.com/alphagov/frontend into travel-advice-index