A forum post whose text is just a YouTube link used the raw link as its
title and had no thumbnail, and the open topic showed only the link.
The topic card now takes the title and picture from the link's preview when
the post opens with a bare link and has no typed title, and the picture goes
through the media proxy like every other remote image (posted image
thumbnails did not before). The open topic gets the same video player or
link card the post gets in chat. A typed title still wins.
YouTube previews come from YouTube's oEmbed endpoint, since the watch page
is too large for the generic scrape to reach its title and image.
