fix: encrypt broadcast lists

it was all the time questionable if not encrypting broadcast lists
rules the issue that recipients may know each other cryptographically.

however, meanwhile with chatmail, unncrypted broadcasts are no longer possible,
and we actively broke workflows eg. from this teacher:
https://support.delta.chat/t/broadcast-funktioniert-nach-update-nicht-meht/3694

this basically reverts commit
7e5907d
which was that time added last-minute and without lots discussions :)

let the students get their homework again :)

src/chat/chat_tests.rs

@@ -2552,7 +2552,7 @@ async fn test_broadcast() -> Result<()> {
         let msg = bob.recv_msg(&sent_msg).await;
         assert_eq!(msg.get_text(), "ola!");
         assert_eq!(msg.subject, "Broadcast list");
-        assert!(!msg.get_showpadlock()); // avoid leaking recipients in encryption data
+        assert!(msg.get_showpadlock());
         let chat = Chat::load_from_db(&bob, msg.chat_id).await?;
         assert_eq!(chat.typ, Chattype::Mailinglist);
         assert_ne!(chat.id, chat_bob.id);

src/mimefactory.rs

@@ -415,12 +415,10 @@ impl MimeFactory {
 
     fn should_force_plaintext(&self) -> bool {
         match &self.loaded {
-            Loaded::Message { chat, msg } => {
-                msg.param
-                    .get_bool(Param::ForcePlaintext)
-                    .unwrap_or_default()
-                    || chat.typ == Chattype::Broadcast
-            }
+            Loaded::Message { msg, .. } => msg
+                .param
+                .get_bool(Param::ForcePlaintext)
+                .unwrap_or_default(),
             Loaded::Mdn { .. } => false,
         }
     }