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/mod.rs

@@ -16,6 +16,7 @@ pub mod federation;
 pub mod globals;
 pub mod key_backups;
 pub mod media;
+pub mod moderation;
 pub mod presence;
 pub mod pusher;
 pub mod resolver;