Fix issue due date edit toggle bug (go-gitea#23723)

Use `toggleElem` instead of jQuery's `fadeToggle`, which can't be caught
by eslint jquery plugin.

Hopefully this could be the last bug for the jQuery show/hide
refactoring.

Author: wxiaoguang

web_src/js/features/repo-issue.js

@@ -78,7 +78,7 @@ function updateDeadline(deadlineString) {
 
 export function initRepoIssueDue() {
   $(document).on('click', '.issue-due-edit', () => {
-    $('#deadlineForm').fadeToggle(150);
+    toggleElem('#deadlineForm');
   });
   $(document).on('click', '.issue-due-remove', () => {
     updateDeadline('');