Reddit image gallery links

imported from https://todo.sr.ht/~luma_inhibitor/cables/75

When a user posts a reddit submission link and it resolves to an image gallery link, PRAW is unable to further expand this. It results in an infinite loop on the gallery link, which will be resolved by #77 (closed).

Either figure out how to further expand into the first image (not posting the gallery link, which doesn't natively expand either), or exclude reddit.com/gallery links from MediaExpand parsing