Fix regression from https://github.com/go-gitea/gitea/pull/24740 where the loading state was not showing because the `oauth-login-image` class was removed. Replaced the Fomantic loader with a pure CSS loader and cleaned up the HTML. Diff: https://github.com/go-gitea/gitea/pull/24788/files?diff=unified&w=1  Co-authored-by: Giteabot <teabot@gitea.io>