Use "small-loading-icon" insead of "btn-octicon is-loading" (#26710)

The "btn-octicon is-loading" was introduced by #21842 , it is only used
by the "Copy Content" button, but the "btn-octicon" selector would
affect too many uncertain elements.

Now there is a general "small-loading-icon" class, so the "btn-octicon
is-loading" could be removed.

web_src/js/features/copycontent.js

@@ -18,7 +18,7 @@ export function initCopyContent() {
     // the text to copy is not in the DOM or it is an image which should be
     // fetched to copy in full resolution
     if (link) {
-      btn.classList.add('is-loading');
+      btn.classList.add('is-loading', 'small-loading-icon');
       try {
         const res = await fetch(link, {credentials: 'include', redirect: 'follow'});
         const contentType = res.headers.get('content-type');
@@ -32,7 +32,7 @@ export function initCopyContent() {
       } catch {
         return showTemporaryTooltip(btn, i18n.copy_error);
       } finally {
-        btn.classList.remove('is-loading');
+        btn.classList.remove('is-loading', 'small-loading-icon');
       }
     } else { // text, read from DOM
       const lineEls = document.querySelectorAll('.file-view .lines-code');