FEATURE: More flexible admin plugin config nav definition (#26254)

This commit changes the API for registering the plugin config
page nav configuration from a server-side to a JS one;
there is no need for it to be server-side.

It also makes some changes to allow for 2 different ways of displaying
navigation for plugin pages, depending on complexity:

* TOP - This is the best mode for simple plugins without a lot of different
  custom configuration pages, and it reuses the grey horizontal nav bar
  already used for admins.
* SIDEBAR - This is better for more complex plugins; likely this won't
  be used in the near future, but it's readily available if needed

There is a new AdminPluginConfigNavManager service too to manage which
plugin the admin is actively viewing, otherwise we would have trouble
hiding the main plugin nav for admins when viewing a single plugin.

app/serializers/admin_plugin_serializer.rb

@@ -16,12 +16,7 @@ class AdminPluginSerializer < ApplicationSerializer
              :commit_hash,
              :commit_url,
              :meta_url,
-             :authors,
-             :admin_config_nav_routes
-
-  def admin_config_nav_routes
-    object.admin_config_nav_routes
-  end
+             :authors
 
   def id
     object.directory_name