Fix for HTML5 validation

Fix for HTML5 validation. The current implementation does only validate HTML4/XHTML due to the use of the "navigation_root_url" metatag. This has now been changed to be link rel="home" instead.

See the discussion: https://github.com/plone/plone.app.search/commit/df828f248e3b49374a591554c46d3ed23636a617#commitcomment-3294637