Don't share all the gallery images on facebook.

Facebook does a lousy job of selecting the best default image to share when several are available. And we want people to visit our site to see all the lovely gallery pictures, not to stick around in Facebook and have the same benefit.

This PR removes the og:image metadata for all but the trail picture, which we definitely _do_ want to share on the 'book. This change only affects galleries, but I've added a test for a regular article too. 

(Requested by @crifmulholland)