Improvements for Content Copy (#21842)

It now supports copying Markdown, SVG and Images (not in Firefox
currently because of lacking
[`ClipboardItem`](https://developer.mozilla.org/en-US/docs/Web/API/ClipboardItem)
support, but can be enabled in `about:config` and works). It will fetch
the data if in a rendered view or when it's an image.

Followup to https://github.com/go-gitea/gitea/pull/21629.

web_src/js/modules/tippy.js

@@ -27,6 +27,7 @@ export function createTippy(target, opts = {}) {
 export function initTooltip(el, props = {}) {
   const content = el.getAttribute('data-content') || props.content;
   if (!content) return null;
+  if (!el.hasAttribute('aria-label')) el.setAttribute('aria-label', content);
   return createTippy(el, {
     content,
     delay: 100,