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?

plugins/chat/spec/plugin_spec.rb

@@ -125,6 +125,24 @@ describe Chat do
     end
   end
 
+  describe "admin plugin serializer extension" do
+    let(:admin) { Fabricate(:admin) }
+    let(:chat_plugin) do
+      Plugin::Instance.parse_from_source(File.join(Rails.root, "plugins", "chat", "plugin.rb"))
+    end
+    let(:serializer) { AdminPluginSerializer.new(chat_plugin, scope: admin.guardian) }
+
+    it "includes all incoming webhooks via :incoming_chat_webhooks" do
+      webhook = Fabricate(:incoming_chat_webhook)
+      expect(serializer.incoming_chat_webhooks).to contain_exactly(webhook)
+    end
+
+    it "includes all chat channels via :chat_channels" do
+      channel = Fabricate(:chat_channel)
+      expect(serializer.chat_channels).to contain_exactly(channel)
+    end
+  end
+
   describe "chat oneboxes" do
     fab!(:chat_channel) { Fabricate(:category_channel) }
     fab!(:user)

plugins/chat/spec/requests/application_controller_spec.rb

@@ -22,7 +22,7 @@ RSpec.describe ApplicationController do
       sign_in(admin)
       get "/latest"
       expect(JSON.parse(preloaded_json["enabledPluginAdminRoutes"])).to include(
-        { "label" => "chat.admin.title", "location" => "chat" },
+        { "label" => "chat.admin.title", "location" => "chat", "use_new_show_route" => false },
       )
     end
   end