DEV: Single admin plugin page for consistent admin plugin UX (#26024)

This commit adds new plugin show routes (`/admin/plugins/:plugin_id`) as we move
towards every plugin having a consistent UI/landing page.

As part of this, we are introducing a consistent way for plugins
to show an inner sidebar in their config page, via a new plugin
API `register_admin_config_nav_routes`

This accepts an array of links with a label/text, and an
ember route. Once this commit is merged we can start the process
of conforming other plugins to follow this pattern, as well
as supporting a single-page version of this for simpler plugins
that don't require an inner sidebar.

Part of /t/122841 internally

plugins/chat/plugin.rb

@@ -132,6 +132,16 @@ after_initialize do
     end
   end
 
+  add_to_serializer(
+    :admin_plugin,
+    :incoming_chat_webhooks,
+    include_condition: -> { self.name == "chat" },
+  ) { Chat::IncomingWebhook.includes(:chat_channel).all }
+
+  add_to_serializer(:admin_plugin, :chat_channels, include_condition: -> { self.name == "chat" }) do
+    Chat::Channel.public_channels
+  end
+
   add_to_serializer(:user_card, :can_chat_user) do
     return false if !SiteSetting.chat_enabled
     return false if scope.user.blank?