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

Tabs do not need to be wrapped in `nav` tags as they are already identified by their role attribute.
Skiplinks do not need to be wrapped in `nav` tags.

Note: needs to be merged with the following pull requests:

https://github.com/alphagov/static/pull/238
https://github.com/alphagov/frontend/pull/328
https://github.com/alphagov/trade-tariff-frontend/pull/75