Alasdair/header footer accessibility updates

@stephensanchez @andywaldrop @talbs https://openedx.atlassian.net/browse/ECOM-345

Updating header to be more accessible, including replacing lists with only 1 item with divs and adding aria tags.
On line 94 of navigation.html there is still a case where the DOM is using a list so we need to keep the li here