Resultify join_securejoin

This gets rid of ChatId::new(0) usage and is generally a nice first
refactoing step.  The complexity of cleanup() unravels nicely.

src/chat.rs

@@ -2701,6 +2701,7 @@ pub(crate) async fn get_chat_cnt(context: &Context) -> usize {
     }
 }
 
+/// Returns a tuple of `(chatid, is_verified, blocked)`.
 pub(crate) async fn get_chat_id_by_grpid(
     context: &Context,
     grpid: impl AsRef<str>,