Securejoin: Fix adding and handling Autocrypt-Gossip headers (#3836)

- If bcc_self is set, gossip headers must be added despite of the number of group members.
- If another device observes Secure-Join, instead of looking for Secure-Join-Fingerprint in
  "vg-member-added"/"vc-contact-confirm" messages it must use keys from Autocrypt-Gossip headers as
  described in the Countermitm doc
  (https://countermitm.readthedocs.io/en/latest/new.html#joining-a-verified-group-secure-join).

src/mimefactory.rs

@@ -689,7 +689,9 @@ impl<'a> MimeFactory<'a> {
                 .fold(message, |message, header| message.header(header));
 
             // Add gossip headers in chats with multiple recipients
-            if peerstates.len() > 1 && self.should_do_gossip(context).await? {
+            if (peerstates.len() > 1 || context.get_config_bool(Config::BccSelf).await?)
+                && self.should_do_gossip(context).await?
+            {
                 for peerstate in peerstates.iter().filter_map(|(state, _)| state.as_ref()) {
                     if peerstate.peek_key(min_verified).is_some() {
                         if let Some(header) = peerstate.render_gossip_header(min_verified) {