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.

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

web_src/js/utils/dom.js

@@ -183,3 +183,14 @@ export function autosize(textarea, {viewportMarginBottom = 0} = {}) {
 export function onInputDebounce(fn) {
   return debounce(300, fn);
 }
+
+// Set the `src` attribute on an element and returns a promise that resolves once the element
+// has loaded or errored. Suitable for all elements mention in:
+// https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/load_event
+export function loadElem(el, src) {
+  return new Promise((resolve) => {
+    el.addEventListener('load', () => resolve(true), {once: true});
+    el.addEventListener('error', () => resolve(false), {once: true});
+    el.src = src;
+  });
+}