ATProto: include link to videos instead of silently dropping them

Bluesky doesn't support videos yet, so we can't bridge them directly like we do with images, but we should do something, probably a link. Example: https://mastodon.social/@jjb/112488173247909514 => https://bsky.app/profile/jjb.mastodon.social.ap.brid.gy/post/3kt4t4wf2x4o2

Thanks @wakest!