Replace ajax with fetch, improve image diff (#27267)

1. Dropzone attachment removal, pretty simple replacement
2. Image diff: The previous code fetched every image twice, once via
`img[src]` and once via `$.ajax`. Now it's only fetched once and a
second time only when necessary. The image diff code was partially
rewritten.

---------

Co-authored-by: Giteabot <teabot@gitea.io>

web_src/js/features/common-global.js

@@ -11,7 +11,7 @@ import {htmlEscape} from 'escape-goat';
 import {showTemporaryTooltip} from '../modules/tippy.js';
 import {confirmModal} from './comp/ConfirmModal.js';
 import {showErrorToast} from '../modules/toast.js';
-import {request} from '../modules/fetch.js';
+import {request, POST} from '../modules/fetch.js';
 
 const {appUrl, appSubUrl, csrfToken, i18n} = window.config;
 
@@ -243,9 +243,8 @@ export function initGlobalDropzone() {
         this.on('removedfile', (file) => {
           $(`#${file.uuid}`).remove();
           if ($dropzone.data('remove-url')) {
-            $.post($dropzone.data('remove-url'), {
-              file: file.uuid,
-              _csrf: csrfToken,
+            POST($dropzone.data('remove-url'), {
+              data: new URLSearchParams({file: file.uuid}),
             });
           }
         });