Add lambda to the validations that use model constants

In this way when we need modify the constants model value in the
model/custom folder, adding lambda it will be possible load the new
values.

spec/models/site_customization/page_spec.rb

@@ -13,4 +13,14 @@ RSpec.describe SiteCustomization::Page, type: :model do
     custom_page = build(:site_customization_page, slug: "as/as*la")
     expect(custom_page).to be_invalid
   end
+
+  it "dynamically validates the valid statuses" do
+    stub_const("#{SiteCustomization::Page}::VALID_STATUSES", %w[custom])
+
+    custom_page.status = "custom"
+    expect(custom_page).to be_valid
+
+    custom_page.status = "published"
+    expect(custom_page).not_to be_valid
+  end
 end