add notification for new post in thread

views/threads/new-post-notification.etlua

@@ -0,0 +1,10 @@
+<div id="new-post-notification" class="new-concept-notification hidden">
+  <div class="new-notification-content">
+    <p>New post in thread!</p>
+    <span class="notification-buttons">
+      <button id="dismiss-new-post-button">Dismiss</button>
+      <a class="linkbutton" id="go-to-new-post-button">View post</a>
+      <button id="unsub-new-post-button">Stop updates</button>
+    </span>
+  </div>
+</div>

views/threads/thread.etlua

@@ -55,4 +55,6 @@
     </span>
   </div>
 </dialog>
+<input type="hidden" id="thread-subscribe-endpoint" value="<%= url_for("sse_thread_updates", {thread_id = thread.id}) %>">
+<% render("views.threads.new-post-notification") %>
 <script src="/static/js/thread.js"></script>