Implement feature #1652 google custom search support. For now we only support the look and feel "results only" and the engine must be set to use that layout. Add a google section to the search providers and search_form and search layout.
Close#1652
Where it makes sense replace asset paths and navigation related paths with `relative_url` filter.
Leave SEO related `<head>` elements and social sharing links as `absolute_url`.
Fixes#1588
