use placeholder for GIFs that can't be loaded still because of CORS

[tomcasaburi]

We don't want to force GIF autoplay in the feed, as it's distracting for some users, so it's usually bad UX. This is why we already try to load the first frame of the GIF, to display it as still image in the feed (until clicked to enlarge and play the GIF).

Some GIFs can't be loaded as still images in the feed, because of CORS. These GIFs are currently being autoplayed in the feed, which is bad UX. Instead, we should use a placeholder for them.