Add ACX substack demo

src/demo.rs

@@ -70,7 +70,15 @@ pub async fn setup_demo_data(pool: &sqlx::SqlitePool) {
     );
     isidore.categorization = vec!["Webcomic".to_string()];
 
-    let feeds = [bbc_news, xkcd, isidore];
+    let mut acx = Feed::new(
+        "Astral Codex Ten".to_string(),
+        "https://www.astralcodexten.com/feed".parse().unwrap(),
+        demo_id,
+    );
+    acx.categorization = vec!["Substack".to_string()];
+
+
+    let feeds = [bbc_news, xkcd, isidore, acx];
 
     for feed in feeds {
         // TODO: This insert logic is substantially the same as Feed::write_to_database.