Quick accessibility fixes

Feedback from an accessibility review showed misuse of the `nav` tag was making the document difficult to read:

https://www.pivotaltracker.com/story/show/47377249

`nav` tags were removed from the footer as being in the footer tag gave enough context.
Skip links didn't need `nav` tags.
The breadcrumb was better suited to being identified by a breadcrumbs ARIA role.

Note: needs to be merged with the following pull requests:

https://github.com/alphagov/frontend/pull/328
https://github.com/alphagov/calendars/pull/31
https://github.com/alphagov/trade-tariff-frontend/pull/75