Indicate video embeds when there's no internet connection

[nolanw]

Steps to reproduce:

1. With an active internet connection, load a few pages of a thread with video embeds in there somewhere.
2. Turn on Airplane Mode.
3. Go next/back to one of the pages with a video embed.

Expected behavior:

Ideally, the video was cached and plays. But next best would be some indication that a video would show there if I had an internet connection.

Observed behavior:

There's a video-embed-sized empty space in the post.