Improve performance of pages_virtual_domain

When exposing Namespace#pages_virtual_domain it eventually delegates
`private_pages?` to `project_feature`, and needs `route` for
`full_path`. Preloading these two associations avoids N+1 queries
problem.

Related to https://gitlab.com/gitlab-org/gitlab/issues/207096.

app/models/namespace.rb

@@ -326,7 +326,10 @@ class Namespace < ApplicationRecord
   end
 
   def pages_virtual_domain
-    Pages::VirtualDomain.new(all_projects_with_pages, trim_prefix: full_path)
+    Pages::VirtualDomain.new(
+      all_projects_with_pages.includes(:route, :project_feature),
+      trim_prefix: full_path
+    )
   end
 
   def closest_setting(name)

spec/models/namespace_spec.rb

@@ -983,6 +983,24 @@ describe Namespace do
           expect(virtual_domain.lookup_paths).not_to be_empty
         end
       end
+
+      it 'preloads project_feature and route' do
+        project2 = create(:project, namespace: namespace)
+        project3 = create(:project, namespace: namespace)
+
+        project.mark_pages_as_deployed
+        project2.mark_pages_as_deployed
+        project3.mark_pages_as_deployed
+
+        virtual_domain = namespace.pages_virtual_domain
+
+        queries = ActiveRecord::QueryRecorder.new { virtual_domain.lookup_paths }
+
+        # 1 to load projects
+        # 1 to preload project features
+        # 1 to load routes
+        expect(queries.count).to eq(3)
+      end
     end
   end