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/membership.rs

@@ -83,9 +83,8 @@ async fn banned_room_check(
 	if let Some(room_id) = room_id {
 		if services.rooms.metadata.is_banned(room_id).await
 			|| services
-				.config
-				.forbidden_remote_server_names
-				.is_match(room_id.server_name().expect("legacy room mxid").host())
+				.moderation
+				.is_remote_server_forbidden(room_id.server_name().expect("legacy room mxid"))
 		{
 			warn!(
 				"User {user_id} who is not an admin attempted to send an invite for or \