Display when a podcast has been removed with an explanation of the reason

[mitchdowney]

We recently had to remove a podcast from Podverse due to a report that it was in violation of copyright. Now when people go to visit the link that previously existed on Podverse for the content, they receive a 404 page.

Ideally we would handle this situation more gracefully, and provide some explanation to the user as to why the content is no longer available. We should probably make this system a bit open-ended, so we can render different reasons for the take-down depending on the content violation.

podverse/podverse-web#1186
podverse/podverse-api#658