Oneboxing large images fails with no visual cue

[nbianca (Bianca)]

@Arkshine

The previous behavior always displayed a placeholder with text saying the image was too large.
Now, because the onebox contains a link, it’s always removed.

There are two ways images can be oneboxed and unfortunately, only one was handled correctly. This should be fixed in:

https://github.com/discourse/discourse/pull/21237