Generate rfc724_mid when creating Message (#6704)

Set `rfc724_mid` in `Message::new()`, `Message::new_text()`, and
`Message::default()` instead of when sending the message. This way the
rfc724 mid can be read in the draft stage which makes it more consistent
for bots. Tests had to be adjusted to create multiple messages to get
unique mid, otherwise core would not send the messages out.

src/mimefactory/mimefactory_tests.rs

@@ -728,6 +728,7 @@ async fn test_selfavatar_unencrypted_signed() {
         .is_some());
 
     // if another message is sent, that one must not contain the avatar
+    let mut msg = Message::new_text("this is the text!".to_string());
     let sent_msg = t.send_msg(chat.id, &mut msg).await;
     let mut payload = sent_msg.payload().splitn(4, "\r\n\r\n");