feat: Do not copy Chat-Version header into outer part

Chat-Version is used sometimes by Sieve filters to move messages to DeltaChat folder:
https://github.com/mailcow/mailcow-dockerized/blob/37beed6ad93f259b97cad41877982bce95295629/data/conf/dovecot/global_sieve_before
This probably prevents notifications to MUAs that don't watch DeltaChat but watch INBOX.

There are however disadvantages to exposing Chat-Version:
1. Spam filters may not like this header and it is difficult or impossible to tell if `Chat-Version`
   plays role in rejecting the message or delivering it into Spam folder. If there is no such header
   visible to the spam filter, this possibility can be ruled out.
2. Replies to chat messages may have no `Chat-Version` but have to be moved anyway.
3. The user may have no control over the Sieve filter, but it comes preconfigured in mailcow, so it
   is not possible to disable it on the client.

Thanks to link2xt for providing this motivation.

Author: iequidoo

src/mimefactory.rs

@@ -947,8 +947,7 @@ impl MimeFactory {
         //
         // These are standard headers such as Date, In-Reply-To, References, which cannot be placed
         // anywhere else according to the standard. Placing headers here also allows them to be fetched
-        // individually over IMAP without downloading the message body. This is why Chat-Version is
-        // placed here.
+        // individually over IMAP without downloading the message body.
         let mut unprotected_headers: Vec<(&'static str, HeaderType<'static>)> = Vec::new();
 
         // Headers that MUST NOT (only) go into IMF header section:
@@ -1063,11 +1062,7 @@ impl MimeFactory {
                             mail_builder::headers::raw::Raw::new("[...]").into(),
                         ));
                     }
-                    "in-reply-to"
-                    | "references"
-                    | "auto-submitted"
-                    | "chat-version"
-                    | "autocrypt-setup-message" => {
+                    "in-reply-to" | "references" | "auto-submitted" | "autocrypt-setup-message" => {
                         unprotected_headers.push(header.clone());
                     }
                     _ => {