Improve branch list UI (#27319) (#27324)

Backport #27319 by @wxiaoguang

1. Put the `"octicon-shield-lock"` into the flex container, then it
doesn't need a separate flex box
2. Remove some unnecessary `gt-df` helpers
3. Make `btn` button has the same flex behavior as `ui button`

![image](https://github.com/go-gitea/gitea/assets/2114189/60ce75f7-7fac-4157-9c42-91c7dee9300e)

![image](https://github.com/go-gitea/gitea/assets/2114189/ea606baf-6f52-41e1-b964-c4840d3b1529)

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/css/modules/button.css

@@ -30,7 +30,7 @@
   color: var(--color-red);
 }
 
-/* btn is a plain button without any opinionated styling */
+/* btn is a plain button without any opinionated styling, it only uses flex for vertical alignment like ".ui.button" in base.css */
 
 .btn {
   background: transparent;