add aria landmark to nav
add aria landmark to nav

Summary:
https://www.w3.org/WAI/GL/wiki/Using_HTML5_nav_element for info on the `role="navigation'`

There is conflicting out there if we should use `role="navigation'` on `<nav>` tagged html, but W3C says yes, so I think it makes sense.  Usually, roles should not be given if the correct html tag is uses.

Reviewed By: fanzeyi

Differential Revision: D30490533

fbshipit-source-id: 81766063bcc849c345d54b64dead98806d99d4e3

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