add reply button functionality

js/thread.js

@@ -0,0 +1,8 @@
+const ta = document.getElementById("post_content");
+
+for (let button of document.querySelectorAll(".reply-button")) {
+  button.addEventListener("click", (e) => {
+    ta.value += button.value;
+    ta.scrollIntoView()
+  })
+}