Fix some UI alignments (#25277)

Fixes: https://github.com/go-gitea/gitea/issues/25282

Fix the problems:

1. The `repo-button-row` had various patches before, this PR makes it
consistent
2. The "Add File" has wrong CSS class "icon", remove it
3. The "Add File" padding was overridden by "!important", fix it by
`.repo-button-row .button.dropdown` with comment
4. The selector `.ui.segments ~ .ui.top.attached.header` is incorrect,
it should use `+`

web_src/css/base.css

@@ -2099,8 +2099,8 @@ a.ui.basic.label:hover {
 }
 
 /* if a .top.attached.header is followed by a .segment, add some margin */
-.ui.segments ~ .ui.top.attached.header,
-.ui.attached.segment ~ .ui.top.attached.header {
+.ui.segments + .ui.top.attached.header,
+.ui.attached.segment + .ui.top.attached.header {
   margin-top: 1rem;
 }