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.
`nav` tags that are in use needed proper labelling through use of either the aria-labelledby or aria-label attributes.