Don't depend on these 3 cleartext headers for the question whether we
download a message.
This PR will waste a bit of bandwidth for people who use the legacy
show_emails option; apart from that, there is no user-visible change
yet. It's a preparation for being able to remove these headers, in order
to further reduce unencrypted metadata.
Removing In-Reply-To and References will be easy; removing Chat-Version
must happen at least one release after the PR here is released, so that
people don't miss messages. Also, maybe some nerds depend on the
Chat-Version header for server-side filtering of messages, but we shall
have this discussion at some other time.
For the question whether a message should be moved, we do still depend
on them; this will be fixed with
https://github.com/chatmail/core/pull/7780.
When both this PR and #7780 are merged, we can stop requesting
Chat-Version header during prefetch.
b94792706a