Vendor jquery.are-you-sure with strict mode fixes (#26901)

Extract from https://github.com/go-gitea/gitea/pull/25940 and because
https://github.com/go-gitea/gitea/pull/26743 does seem to need more
work.

This will be required if we are to run our JS in [strict
mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode).

Previously, the two variables `$fields` and `$dirtyForms` polluted
`window`:

<img width="1145" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/e0270a0e-b881-4ed7-9cc4-e9ab25c0a2bc">

web_src/js/features/common-global.js

@@ -1,5 +1,5 @@
 import $ from 'jquery';
-import 'jquery.are-you-sure';
+import '../vendor/jquery.are-you-sure.js';
 import {clippie} from 'clippie';
 import {createDropzone} from './dropzone.js';
 import {initCompColorPicker} from './comp/ColorPicker.js';