Mark links that redirect to a new domain as non-embeddable

It is possible for users to put "Short URLs" into content links which simply redirect to a page whose X-Frame-Options header is set to SAMEORIGIN, however we try and embed it because the link shortener site doesn't have that header set.

To resolve this issue, and also other moderation issues that can arise from cross-domain redirects (e.g., we cannot moderate the target host of links to "safe sites" because they can redirect anywhere), we should probably just not embed or generate previews for links that redirect to a new domain.