Add a config option to sign all messages with Autocrypt header (#3986)

Although it does a little for security, it will help to protect from unwanted server-side
modifications and bugs. And now we have a time to test "multipart/signed" messages compatibility
with other MUAs.

src/context.rs

@@ -763,6 +763,12 @@ impl Context {
                 .await?
                 .unwrap_or_default(),
         );
+        res.insert(
+            "sign_unencrypted",
+            self.get_config_int(Config::SignUnencrypted)
+                .await?
+                .to_string(),
+        );
 
         res.insert(
             "debug_logging",