refactor: Centralize server forbidden checks into moderation module

This moves all checks related to `forbidden_remote_server_names`,
`forbidden_remote_room_directory_server_names` and
`prevent_media_downloads_from` to a new `moderation` module.
This is useful for implementing more complicated logic globally.
Mostly the changes from #673, but is also relevant for #750

src/service/federation/execute.rs

@@ -64,13 +64,7 @@ where
 		return Err!(Config("allow_federation", "Federation is disabled."));
 	}
 
-	if self
-		.services
-		.server
-		.config
-		.forbidden_remote_server_names
-		.is_match(dest.host())
-	{
+	if self.services.moderation.is_remote_server_forbidden(dest) {
 		return Err!(Request(Forbidden(debug_warn!("Federation with {dest} is not allowed."))));
 	}