feat: send Chat-Verified headers in 1:1 chats

Chat-Verified is going to be useful to upgrade one-way verification
to bidirectional verification.

src/mimefactory.rs

@@ -924,9 +924,7 @@ impl<'a> MimeFactory<'a> {
         let mut meta_part = None;
 
         let send_verified_headers = match chat.typ {
-            // In single chats, the protection status isn't necessarily the same for both sides,
-            // so we don't send the Chat-Verified header:
-            Chattype::Single => false,
+            Chattype::Single => true,
             Chattype::Group => true,
             // Mailinglists and broadcast lists can actually never be verified:
             Chattype::Mailinglist => false,