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/api/client/message.rs

@@ -274,9 +274,8 @@ pub(crate) async fn is_ignored_pdu(
 	let ignored_type = IGNORED_MESSAGE_TYPES.binary_search(&pdu.kind).is_ok();
 
 	let ignored_server = services
-		.config
-		.forbidden_remote_server_names
-		.is_match(pdu.sender().server_name().host());
+		.moderation
+		.is_remote_server_forbidden(pdu.sender().server_name());
 
 	if ignored_type
 		&& (ignored_server || services.users.user_is_ignored(&pdu.sender, user_id).await)