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>

tests/e2e/example.test.e2e.js

@@ -23,7 +23,7 @@ test('Test Register Form', async ({page}, workerInfo) => {
   await page.click('form button.ui.primary.button:visible');
   // Make sure we routed to the home page. Else login failed.
   await expect(page.url()).toBe(`${workerInfo.project.use.baseURL}/`);
-  await expect(page.locator('.dashboard-navbar span>img.ui.avatar')).toBeVisible();
+  await expect(page.locator('.secondary-nav span>img.ui.avatar')).toBeVisible();
   await expect(page.locator('.ui.positive.message.flash-success')).toHaveText('Account was successfully created. Welcome!');
 
   save_visual(page);