fix: Don't change webxdc self-addr when saving and loading draft (#6854)

Fix https://github.com/chatmail/core/issues/6621; I also tested on
Android that the webxdc self-addr actually stays the same when staging a
draft and then sending.

Follow-up to https://github.com/chatmail/core/pull/6704; #6704 made sure
that the webxdc self-addr doesn't change when creating a message and
then sending it. This PR here makes sure that the rfc724_mid (which is
needed to compute the self-addr) is saved when setting a draft, so that
it's loaded properly after a call to get_draft().

cc @adbenitez @r10s @Septias

src/webxdc/webxdc_tests.rs

@@ -2210,6 +2210,7 @@ async fn test_self_addr_consistency() -> Result<()> {
         include_bytes!("../../test-data/webxdc/minimal.xdc"),
     )?;
     alice_chat.set_draft(alice, Some(&mut instance)).await?;
+    let mut instance = alice_chat.get_draft(alice).await?.unwrap();
     let self_addr = instance.get_webxdc_self_addr(alice).await?;
     let sent = alice.send_msg(alice_chat, &mut instance).await;
     let db_msg = Message::load_from_db(alice, sent.sender_msg_id).await?;