feat: Use IMAP APPEND command to upload sync messages (#5845)

Why:
- With IMAP APPEND we can upload messages directly to the DeltaChat folder (for non-chatmail
  accounts).
- We can set the `\Seen` flag immediately so that if the user has other MUA, it doesn't alert about
  a new message if it's just a sync message (there were several such reports on the support
  forum). Though this also isn't useful for chatmail.
- We don't need SMTP envelope and overall remove some overhead on processing sync messages.

src/smtp.rs

@@ -485,7 +485,6 @@ pub(crate) async fn send_smtp_messages(context: &Context, connection: &mut Smtp)
     let ratelimited = if context.ratelimit.read().await.can_send() {
         // add status updates and sync messages to end of sending queue
         context.flush_status_updates().await?;
-        context.send_sync_msg().await?;
         false
     } else {
         true