Update Media lib to support Reddit image galleries

[mrbellek]

Reddit allow users to post multiple images in a post. This results in the media link to be something like 'reddit.com/gallery/tvptpt', but the media module displays this as 'crosslink' and will not attach the pictures. The module must be updated to properly detect a gallery, pick up to 4 random pictures from it, and attach those to the tweet.

This was implemented earlier for posts that linked to Imgur galleries, but since Reddit has implemented their own version of galleries, those aren't used anymore.