Display warning when user try to rename default branch (#24512)

Follow #24380

It's better to warn users when they try to rename the default branch.


![image](https://user-images.githubusercontent.com/33891828/236107929-c8ac2854-dd0f-4500-a0a7-800c8fe48861.png)

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Co-authored-by: techknowlogick <matti@mdranta.net>
Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>

web_src/js/features/repo-branch.js

@@ -1,4 +1,5 @@
 import $ from 'jquery';
+import {toggleElem} from '../utils/dom.js';
 
 export function initRepoBranchButton() {
   initRepoCreateBranchButton();
@@ -31,6 +32,10 @@ function initRepoRenameBranchButton() {
     const oldBranchName = $(this).attr('data-old-branch-name');
     $modal.find('input[name=from]').val(oldBranchName);
 
+    // display the warning that the branch which is chosen is the default branch
+    const $warn = $modal.find('.default-branch-warning');
+    toggleElem($warn, $(this).attr('data-is-default-branch') === 'true');
+
     const $text = $modal.find('[data-rename-branch-to]');
     $text.text($text.attr('data-rename-branch-to').replace('%s', oldBranchName));
   });