Improving mailing lists and fixing things:

Remove additional chat type MailingList, make them GroupLists with a special param, make can_send() return false, code style and deduplication

src/param.rs

@@ -117,6 +117,8 @@ pub enum Param {
 
     /// For MDN-sending job
     MsgId = b'I',
+
+    MailingList = b't'
 }
 
 /// Possible values for `Param::ForcePlaintext`.