Support for PageMaps

[DannyT]

As a result of an issue which came up on another site, it would be good for us to support Google's "PageMaps" functionality:

https://support.google.com/customsearch/answer/1626955?hl=en

The specific issue we had on the other site was Google search results on mobile show a thumbnail which none of the usual metatags seemed to impact. The PageMaps functionality can be used to determine this but there is also a metatag for a simpler solution to that specific issue (see bottom of this page)