Header: Made aria-controls refer to an ID instead of a class

The ```aria-controls``` attribute that was added to the menu-toggle button in https://github.com/Automattic/_s/commit/e142af3a2720e38179e294181562c22842a68c3e refers to the class of the primary menu. According to the specs it should refer to an ID (http://www.w3.org/TR/wai-aria/states_and_properties#aria-controls).

Ref #545