feat: Always move outgoing auto-generated messages to the mvbox

Recently there are many questions on the Delta Chat forum why some unexpected encrypted messages
appear in Inbox. Seems they are mainly sync messages, though that also obviously happens to
SecureJoin messages. Anyway, regardless of the `MvboxMove` setting, auto-generated outgoing messages
should be moved to the DeltaChat folder so as not to complicate co-using Delta Chat with other MUAs.

src/headerdef.rs

@@ -11,6 +11,7 @@ pub enum HeaderDef {
     Date,
     From_,
     To,
+    AutoSubmitted,
 
     /// Carbon copy.
     Cc,