show hint how to restore saved-messages

the saved-messages can be deleted as any other chat;
if the feature is unneeded, this probably also makes some sense.

however, if saved-messages was deleted accidentally (yeah, i've seen that :),
it is not intuitive to see how the saved-messages feature can be restored.

this change just drops a note about how-to-restore to the device chat.

deltachat-ffi/deltachat.h

@@ -4988,6 +4988,7 @@ void dc_event_unref(dc_event_t* event);
 #define DC_STR_PROTECTION_ENABLED         88
 #define DC_STR_PROTECTION_DISABLED        89
 #define DC_STR_REPLY_NOUN                 90 /* eg. "Reply", used in summaries, a noun, not a verb (not: "to reply") */
+#define DC_STR_SELF_DELETED_MSG_BODY      91
 
 /*
  * @}

src/chat.rs

@@ -337,7 +337,7 @@ impl ChatId {
         );
         /* Up to 2017-11-02 deleting a group also implied leaving it, see above why we have changed this. */
 
-        let _chat = Chat::load_from_db(context, self).await?;
+        let chat = Chat::load_from_db(context, self).await?;
         context
             .sql
             .execute(
@@ -373,6 +373,17 @@ impl ChatId {
         let j = job::Job::new(Action::Housekeeping, 0, Params::new(), 10);
         job::add(context, j).await;
 
+        if chat.is_self_talk() {
+            let mut msg = Message::new(Viewtype::Text);
+            msg.text = Some(
+                context
+                    .stock_str(StockMessage::SelfDeletedMsgBody)
+                    .await
+                    .into(),
+            );
+            add_device_msg(&context, None, Some(&mut msg)).await?;
+        }
+
         Ok(())
     }
 

src/stock.rs

@@ -244,6 +244,10 @@ pub enum StockMessage {
     // used in summaries, a noun, not a verb (not: "to reply")
     #[strum(props(fallback = "Reply"))]
     ReplyNoun = 90,
+
+    #[strum(props(fallback = "You deleted the \"Saved messages\" chat.\n\n\
+                    To use the \"Saved messages\" feature again, create a new chat with yourself."))]
+    SelfDeletedMsgBody = 91,
 }
 
 /*