FEATURE: Allow onebox images to be used as topic thumbnails (#12050)

Still excludes GitHub avatars. Those were the original reason for adding
this broad exclusion. Context at https://meta.discourse.org/t/165713/4

If we find more oneboxes which are unsuitable for thumbnails, we can add
them to this selector.

lib/cooked_post_processor.rb

@@ -209,8 +209,8 @@ def extract_images_for_post
     @doc.css("img.site-icon") -
     # minus onebox avatars
     @doc.css("img.onebox-avatar") -
-    # minus small onebox images (large images are .aspect-image-full-size)
-    @doc.css(".onebox .aspect-image img")
+    # minus github onebox profile images
+    @doc.css(".onebox.githubfolder img")
   end
 
   def oneboxed_images