Determine how much of systemPageAttachments() is necessary in BareHtmlPageRenderer

Added the related issue where the call to system_page_attachments was first added to BareHtmlPageRenderer

I've had a look at everything this function does and broken it down below. From what I can tell most of it is still required, except maybe "Attach libraries used by this theme" and "Handle setting the "active" class on links". Would be good to get a second opinion.

// Ensure the same CSS is loaded in
// \Drupal\Core\Theme\ThemePreprocess::preprocessMaintenancePage().
$page['#attached']['library'][] = 'system/base';
if (\Drupal::service('router.admin_context')->isAdminRoute()) {
  $page['#attached']['library'][] = 'system/admin';
}

I'm not sure if this is attaching it for maintenance pages, or for the installer to match maintenance pages. If it's the former, well ThemePreprocess::preprocessMaintenancePage already adds these, so I'll assume it's the latter. And Claro already depends on system/admin so we get these attached on the installer already. But I guess the installer could use a different theme that doesn't have that dependency, so this would be required.

// Attach libraries used by this theme.
$active_theme = \Drupal::theme()->getActiveTheme();
foreach ($active_theme->getLibraries() as $library) {
  $page['#attached']['library'][] = $library;
}

Without this I still get theme libraries attached on the installer, e.g. the first one that xdebug sees is core/drupal.message and I see /core/themes/claro/css/components/messages.css in the page markup. But maintenance pages have no styles without this.

// Attach favicon.
if (theme_get_setting('features.favicon')) {
  $favicon = theme_get_setting('favicon.url');
  $type = theme_get_setting('favicon.mimetype');
  $page['#attached']['html_head_link'][][] = [
    'rel' => 'icon',
    'href' => UrlHelper::stripDangerousProtocols($favicon),
    'type' => $type,
  ];
}

Without this there is no favicon on the installer.

// Attach default meta tags.
// ...
foreach ($meta_default as $key => $value) {
  $page['#attached']['html_head'][] = [$value, $key];
}

As above.

// Handle setting the "active" class on links by:
// - loading the active-link library if the current user is authenticated;
// - applying a response filter if the current user is anonymous.
// @see \Drupal\Core\Link
// @see \Drupal\Core\Utility\LinkGenerator::generate()
// @see \Drupal\Core\Theme\ThemePreprocess::preprocessLinks()
// @see \Drupal\Core\EventSubscriber\ActiveLinkResponseFilter
$page['#cache']['contexts'][] = 'user.roles:authenticated';
if (\Drupal::currentUser()->isAuthenticated()) {
  $page['#attached']['library'][] = 'core/drupal.active-link';
}

Unless I'm overlooking something, by definition it's impossible for the user to be authenticated at install time.

Possibly a duplicate of #3428339: Remove calls to system_page_attachments() actually. Although maybe that can be repurposed to just moving the library from system to core.

Can the summary be flushed out some please, sorry to be that guy.

I think we need @catch to weigh in here. I think it might be better to focus on the work in #3428339: Remove calls to system_page_attachments() and see if there's anything left to do here.