Defer Discord interactions to increase timeout

Occasionally there's an error report from the Discord bot about an “unknown interaction”. According to <https://stackoverflow.com/a/68774492> this may be caused by the response taking more than 3 seconds, and calling [`defer_ephemeral`](https://docs.rs/serenity/latest/serenity/model/application/struct.CommandInteraction.html#method.defer_ephemeral) first may fix this.