Add thumbnail meta tag to plugin pages for custom search

[joostdevalk]

To improve the internal search, we should add the following to plugin pages:

<meta name="thumbnail" content="http://example/foo.jpg" />

Right now Google picks up on the OpenGraph tag on those pages that has the header image in it. The header image is fine for sharing those pages on Facebook and other social platforms, but in the search results, the plugin icon would actually be a far better experience as it fits way better.

[joostdevalk]

The Google doc for this specific GCSE feature: ​https://support.google.com/customsearch/answer/1626955

[ocean90]

Added in [dotorg11494].