FIX: Always preload admin plugin list for admin in sidebar (#25606)

When we show the links to installed plugins in the admin
sidebar (for plugins that have custom admin routes) we were
previously only doing this if you opened /admin, not if you
navigated there from the main forum. We should just always
preload this data if the user is admin.

This commit also changes `admin_sidebar_enabled_groups` to
not be sent to the client as part of ongoing efforts to
not check groups on the client, since not all a user's groups
may be serialized.

spec/requests/application_controller_spec.rb

@@ -1308,6 +1308,7 @@ RSpec.describe ApplicationController do
             "topicTrackingStates",
             "topicTrackingStateMeta",
             "fontMap",
+            "enabledPluginAdminRoutes",
           ],
         )
       end
@@ -1320,6 +1321,11 @@ RSpec.describe ApplicationController do
           DiscourseFonts.fonts.filter { |f| f[:variants].present? }.map { |f| f[:key] },
         )
       end
+
+      it "has correctly loaded enabledPluginAdminRoutes" do
+        get "/latest"
+        expect(JSON.parse(preloaded_json["enabledPluginAdminRoutes"])).to eq([])
+      end
     end
   end
 end