videos: move API's JSON structure to a dedicated module

src/invidious/playlists.cr

@@ -56,7 +56,7 @@ struct PlaylistVideo
       json.field "authorUrl", "/channel/#{self.ucid}"
 
       json.field "videoThumbnails" do
-        generate_thumbnails(json, self.id)
+        Invidious::JSONify::APIv1.thumbnails(json, self.id)
       end
 
       if index