Although we can't add URLs yet, I'm adding this now after discussing it a couple of weeks back with warp...

The relationships which have whitelists should only allow those sites. Those are:
http://wiki.musicbrainz.org/Has_Cover_Art_At_Relationship_Type
http://wiki.musicbrainz.org/Has_Lyrics_At_Relationship_Type
http://wiki.musicbrainz.org/Has_Score_At_Relationship_Type

This mainly applies to adding URLs, but it's also possible to get URLs which aren't allowed into the database by editing an existing URL or relationship. It would be nice if we could prevent that as well.