Add option to toggle publishing for mailing list replies

For some lists, such as release announcement lists, you may not want
replies to posts published.

src/config.rs

@@ -65,6 +65,8 @@ pub struct GitlabWebhookConfig {
 #[derive(Clone, Deserialize)]
 pub struct MailListConfig {
     pub name: String,
+    #[serde(default = "default_true")]
+    pub publish_on_replies: bool,
     #[serde(default)]
     pub rooms: Vec<OwnedRoomOrAliasId>,
 }
@@ -89,6 +91,10 @@ pub struct Config {
     pub mail_archive: Option<MailArchiveConfig>,
 }
 
+fn default_true() -> bool {
+    true
+}
+
 fn load_blocking(path: &String) -> anyhow::Result<Config> {
     let f = File::open(path)?;
     let r = BufReader::new(f);