CSS: Fix thumbnails' aspect ratio to prevent CLS (#4278)

Force the thumbnails aspect ratio to 16/9 in order to prevent Cumulative Layout
Shifting (CLS) from hapenning during lazy loading.

It also fixes the problematic, taller thumbnails that Youtube returns for
playlists.

Closes issue 4002

assets/css/default.css

@@ -197,6 +197,7 @@ img.thumbnail {
   display: block; /* See: https://stackoverflow.com/a/11635197 */
   width: 100%;
   object-fit: cover;
+  aspect-ratio: 16 / 9;
 }
 
 .thumbnail-placeholder {