feat: Disable MDNs for bots by default

- To avoid receiving undecryptable MDNs by bots and replying to them if the bot's key changes.
- MDNs from bots don't look useful in general, usually the user expects some reply from the bot, not
  just that the message is read.

src/mimefactory.rs

@@ -186,7 +186,7 @@ impl MimeFactory {
 
             if !msg.is_system_message()
                 && msg.param.get_int(Param::Reaction).unwrap_or_default() == 0
-                && context.get_config_bool(Config::MdnsEnabled).await?
+                && context.mdns_enabled().await?
             {
                 req_mdn = true;
             }