Prevent multiple codecomments to be submitted

- Utilize the existing `is-loading` class to prevent the submit code
from running twice or more.
- The `is-loading` class gives us a nice loading indicator when added to
the form.
- Resolves https://codeberg.org/forgejo/forgejo/issues/1476

web_src/js/features/repo-diff.js

@@ -45,6 +45,9 @@ function initRepoDiffConversationForm() {
     e.preventDefault();
 
     const $form = $(e.target);
+    if ($form.hasClass('is-loading')) return;
+    $form.addClass('is-loading');
+
     const $textArea = $form.find('textarea');
     if (!validateTextareaNonEmpty($textArea)) {
       return;