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.
@AlasdairSwan and @cptvitamin, should this have a role of navigation given that its not the primary navigation for the view? Is it okay to A) use it for secondary nav and B) more than once in the DOM/view?