Replace sections with role=main with main element
Fix tests
Merge remote-tracking branch 'origin/master' into the-one-main-main
Fix for inherited main#content styles

https://github.com/alphagov/static/pull/294#pullrequestreviewcomment-5502045
Merge pull request #406 from alphagov/the-one-main-main

Replace sections with role=main with main element