[ticket/12845] Fix invalid ARIA role for breadcrumbs

The current ARIA roles assigned to the breadcrumbs are invalid according to the HTML5 aria spec (not allowed on `li`). Since the navbar already has `role=menubar`, this should cover the functionality adequately for screen-readers (for now).

https://tracker.phpbb.com/browse/PHPBB3-12845
PHPBB3-12845