VideoPress: Private videos do not display anything when rendered for email

When VideoPress videos are rendered on email (see D126650-code), there is no distinction for public and private videos. When the video is private, its poster image is also private and the email client fails to load it due to a `403` response. This makes the video end up being rendered as an empty space:


We need to check if the video is private and, if so, display _something else_ instead. Either a static placeholder or a simple text link.

Start with the email-rendering implementation referenced by D126650-code and trace how VideoPress videos and poster images are rendered. Confirm the private-video case against an email client or rendering test, then make the result show either a static placeholder or a simple text link instead of an empty space.

Written by the indexing model from the issue text.