Fix download buttons on branches page (#30147)

Fixes https://github.com/go-gitea/gitea/issues/30143, regression from
https://github.com/go-gitea/gitea/pull/29920.

We have `.button` on the repo page, but on the branch page it's a
`.btn`. Eventually we should find a solution to have a single button
class but until then this solution should be acceptable.

web_src/css/modules/animations.css

@@ -13,6 +13,7 @@
   opacity: 0.3;
 }
 
+.btn.is-loading > *,
 .button.is-loading > * {
   opacity: 0;
 }