fix: Only add "member added/removed" messages if they actually do that (#5992)

There were many cases in which "member added/removed" messages were added to chats even if they
actually do nothing because a member is already added or removed. But primarily this fixes a
scenario when Alice has several devices and shares an invite link somewhere, and both their devices
handle the SecureJoin and issue `ChatGroupMemberAdded` messages so all other members see a
duplicated group member addition.

src/chat.rs

@@ -5371,7 +5371,7 @@ mod tests {
 
         // Eventually, first removal message arrives.
         // This has no effect.
-        bob.recv_msg(&remove1).await;
+        bob.recv_msg_trash(&remove1).await;
         assert_eq!(get_chat_contacts(&bob, bob_chat_id).await?.len(), 2);
         Ok(())
     }