Twitter Card and OpenGraph Support

[pjdufour]

Robust support for Twitter Card (https://dev.twitter.com/cards/overview) and OpenGraph (http://ogp.me/) is still missing. There is some open graph support but it's not fully enabled. Twitter Card and Open Graph are extremely important for a richer social media experience when sharing links, since they automatically add thumbnails, description text, etc. when you click on a "share on twitter" button.

For reference, see PR that added support to GeoNode recently at https://github.com/GeoNode/geonode/pull/2201

Once live, you can test Twitter Card support at: https://cards-dev.twitter.com/validator.