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/securejoin.rs

@@ -109,7 +109,7 @@ pub async fn get_securejoin_qr(context: &Context, group: Option<ChatId>) -> Resu
             context
                 .sync_qr_code_tokens(Some(chat.grpid.as_str()))
                 .await?;
-            context.scheduler.interrupt_smtp().await;
+            context.scheduler.interrupt_inbox().await;
         }
         format!(
             "OPENPGP4FPR:{}#a={}&g={}&x={}&i={}&s={}",
@@ -124,7 +124,7 @@ pub async fn get_securejoin_qr(context: &Context, group: Option<ChatId>) -> Resu
         // parameters used: a=n=i=s=
         if sync_token {
             context.sync_qr_code_tokens(None).await?;
-            context.scheduler.interrupt_smtp().await;
+            context.scheduler.interrupt_inbox().await;
         }
         format!(
             "OPENPGP4FPR:{}#a={}&n={}&i={}&s={}",