Strict typing for system messages; add a way to rename group.

src/routes/channels/delete_channel.rs

@@ -102,8 +102,7 @@ pub async fn req(user: User, target: Ref) -> Result<()> {
             Message::create(
                 "00000000000000000000000000".to_string(),
                 id.clone(),
-                // ! FIXME: make a schema for this
-                format!("{{\"type\":\"user_left\",\"id\":\"{}\"}}", user.id),
+                Content::SystemMessage(SystemMessage::UserLeft { id: user.id })
             )
             .publish(&target)
             .await