Introduce .secondary-nav and handle .page-content spacing universally (#29982)

Fixes: https://github.com/go-gitea/gitea/issues/29981. Introduce
`.secondary-nav` as a universal way for styling and margin adjustments
inside `.page-content`.

If the first child of `.page-content` is `.secondary-nav`, we add margin
below it, otherwise we add padding to the first child. Notable changes:

- `--color-header-wrapper` is replaced with `--color-secondary-nav-bg`.
- `navbar` class is removed.

---------

Co-authored-by: Giteabot <teabot@gitea.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/header.tmpl

@@ -1,4 +1,4 @@
-<div class="header-wrapper">
+<div class="secondary-nav">
 {{with .Repository}}
 	<div class="ui container">
 		<div class="repo-header">
@@ -128,7 +128,7 @@
 		{{if .IsGenerated}}<div class="fork-flag">{{ctx.Locale.Tr "repo.generated_from"}} <a href="{{(.TemplateRepo ctx).Link}}">{{(.TemplateRepo ctx).FullName}}</a></div>{{end}}
 	</div>
 {{end}}
-	<overflow-menu class="ui container secondary pointing tabular top attached borderless menu navbar tw-pt-0 tw-my-0">
+	<overflow-menu class="ui container secondary pointing tabular top attached borderless menu tw-pt-0 tw-my-0">
 		{{if not (or .Repository.IsBeingCreated .Repository.IsBroken)}}
 			<div class="overflow-menu-items">
 				{{if .Permission.CanRead $.UnitTypeCode}}