This distinguishes between YouTube Videos and Image Previews by adding
a particular “youtube-video” class to the preview along with changing
the title to the video ID rather than the link. This serves to allow
the lightbox to ID when a lightbox preview should be treated like a
YouTube video rather than an image preview.
This also modifies the tests in bug down to expect a youtube-video class
along with the title to just be the video ID on YouTube rather than the
entire URL link.