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/config.rs

@@ -589,6 +589,12 @@ impl Context {
             && !self.get_config_bool(Config::Bot).await?)
     }
 
+    /// Returns whether sync messages should be uploaded to the mvbox.
+    pub(crate) async fn should_move_sync_msgs(&self) -> Result<bool> {
+        Ok(self.get_config_bool(Config::MvboxMove).await?
+            || !self.get_config_bool(Config::IsChatmail).await?)
+    }
+
     /// Returns whether MDNs should be requested.
     pub(crate) async fn should_request_mdns(&self) -> Result<bool> {
         match self.get_config_bool_opt(Config::MdnsEnabled).await? {
@@ -792,7 +798,7 @@ impl Context {
         {
             return Ok(());
         }
-        self.scheduler.interrupt_smtp().await;
+        self.scheduler.interrupt_inbox().await;
         Ok(())
     }
 
@@ -1175,7 +1181,7 @@ mod tests {
         let status = "Synced via usual message";
         alice0.set_config(Config::Selfstatus, Some(status)).await?;
         alice0.send_sync_msg().await?;
-        alice0.pop_sent_msg().await;
+        alice0.pop_sent_sync_msg().await;
         let status1 = "Synced via sync message";
         alice1.set_config(Config::Selfstatus, Some(status1)).await?;
         tcm.send_recv(alice0, alice1, "hi Alice!").await;
@@ -1199,7 +1205,7 @@ mod tests {
             .set_config(Config::Selfavatar, Some(file.to_str().unwrap()))
             .await?;
         alice0.send_sync_msg().await?;
-        alice0.pop_sent_msg().await;
+        alice0.pop_sent_sync_msg().await;
         let file = alice1.dir.path().join("avatar.jpg");
         let bytes = include_bytes!("../test-data/image/avatar1000x1000.jpg");
         tokio::fs::write(&file, bytes).await?;