add notification for new post in thread

js/thread.js

@@ -35,4 +35,49 @@
       form.action = `/post/${postId}/delete`
     })
   }
+  
+  let newPostSubscription = null;
+  
+  function hideNotification() {
+    const notification = document.getElementById('new-post-notification');
+    notification.classList.add('hidden');
+  }
+  
+  function showNewPostNotification(url) {
+    const notification = document.getElementById("new-post-notification");
+    
+    notification.classList.remove("hidden");
+    
+    document.getElementById("dismiss-new-post-button").onclick = () => {
+      hideNotification();
+      reconnectSSE();
+    }
+    
+    document.getElementById("go-to-new-post-button").href = url;
+    
+    document.getElementById("unsub-new-post-button").onclick = () => {
+      hideNotification()
+    }
+  }
+  
+  function reconnectSSE() {
+    if (newPostSubscription) newPostSubscription.close();
+    
+    const threadEndpoint = document.getElementById("thread-subscribe-endpoint").value;
+    newPostSubscription = new EventSource(threadEndpoint);
+    newPostSubscription.onerror = (e) => {
+      console.error(e);
+    };
+    newPostSubscription.addEventListener("new_post_url", (e) => {
+      showNewPostNotification(e.data);
+      newPostSubscription.close();
+    })
+  }
+  window.addEventListener('beforeunload', () => {
+    if(newPostSubscription)
+    {
+      newPostSubscription.close();
+    }
+  });
+  reconnectSSE();
 }