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/chat.rs

@@ -995,6 +995,7 @@ impl ChatId {
                 transaction.execute(
                     "INSERT INTO msgs (
                  chat_id,
+                 rfc724_mid,
                  from_id,
                  timestamp,
                  type,
@@ -1004,9 +1005,10 @@ impl ChatId {
                  param,
                  hidden,
                  mime_in_reply_to)
-         VALUES (?,?,?,?,?,?,?,?,?,?);",
+         VALUES (?,?,?,?,?,?,?,?,?,?,?);",
                     (
                         self,
+                        &msg.rfc724_mid,
                         ContactId::SELF,
                         time(),
                         msg.viewtype,