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/less/animations.less

@@ -33,6 +33,12 @@
   height: var(--height-loading);
 }
 
+.btn-octicon.is-loading::after {
+  border-width: 2px;
+  height: 1.25rem;
+  width: 1.25rem;
+}
+
 code.language-math.is-loading::after {
   padding: 0;
   border-width: 2px;