DEV: Open theme settings objects editor from admin customize theme page (#26006)

Why this change?

The `/admin/customize/themes/:id/schema/name` route is a work in
progress but we want to be able to start navigating to it from the
`/admin/customize/themes/:id` route.

What does this change do?

1. Move `adminCustomizeThemes.schema` to a child route of
   `adminCustomizeThemes.show`. This is because we need the model
   from the parent route and if it isn't a child route we end up
   having to load the theme model again from the server.

1. Add the `objects_schema` attribute to `ThemeSettingsSerializer`

1. Refactor `SiteSettingComponent` to be able to render a button
   so that we don't have to hardcode the button rendering into the
   `SiteSettings::String` component

lib/theme_settings_manager/objects.rb

@@ -14,4 +14,8 @@ class ThemeSettingsManager::Objects < ThemeSettingsManager
     theme.reload
     record.json_value
   end
+
+  def schema
+    @opts[:schema]
+  end
 end