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/utils.js

@@ -128,3 +128,14 @@ export function decodeURLEncodedBase64(base64url) {
     .replace(/_/g, '/')
     .replace(/-/g, '+'));
 }
+
+const domParser = new DOMParser();
+const xmlSerializer = new XMLSerializer();
+
+export function parseDom(text, contentType) {
+  return domParser.parseFromString(text, contentType);
+}
+
+export function serializeXml(node) {
+  return xmlSerializer.serializeToString(node);
+}