Make footer the same width as the header
Make footer the same width as the header

In modern browsers the header and the footer were no longer the same
size. This makes them the same. Also swapped a `media-down` usage to
`media` for future proofing.

quis/govuk_template,daveraine/govuk_template,froddd/govuk_template,bitzesty/govuk_template,daveraine/govuk_template,froddd/govuk_template,alphagov/govuk_template,daveraine/govuk_template,robinwhittleton/govuk_template,alphagov/govuk_template,jonheslop/govuk_template,froddd/govuk_template,jonheslop/govuk_template,jonheslop/govuk_template,quis/govuk_template,jonheslop/govuk_template,quis/govuk_template,alphagov/govuk_template,robinwhittleton/govuk_template,daveraine/govuk_template,robinwhittleton/govuk_template,quis/govuk_template,alphagov/govuk_template,froddd/govuk_template,bitzesty/govuk_template,quis/govuk_template,bitzesty/govuk_template,froddd/govuk_template,robinwhittleton/govuk_template,jonheslop/govuk_template,alphagov/govuk_template,robinwhittleton/govuk_template