Improve branch list UI (#27319)

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)

web_src/css/base.css

@@ -2242,6 +2242,7 @@ table th[data-sortt-desc] .svg {
   right: 2em;
 }
 
+.btn,
 .ui.ui.button,
 .ui.ui.dropdown,
 .ui.ui.label,

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;