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>

web_src/css/repo/header.css

@@ -1,4 +1,8 @@
-.header-wrapper .fork-flag {
+.repository .secondary-nav {
+  padding-top: 12px;
+}
+
+.repository .secondary-nav .fork-flag {
   margin-top: 0.5rem;
   font-size: 12px;
 }
@@ -63,8 +67,3 @@
 .repo-buttons .ui.labeled.button.disabled > .button {
   pointer-events: none !important;
 }
-
-.repository .header-wrapper {
-  padding-top: 12px;
-  background-color: var(--color-header-wrapper);
-}