Add and apply RSpec/Rails/NegationBeValid rule

This rule was added in rubocop-rspec 2.23.0. We were already applying it
most of the time.

spec/models/site_customization/page_spec.rb

@@ -11,7 +11,7 @@ RSpec.describe SiteCustomization::Page do
 
   it "is invalid if slug has symbols" do
     custom_page = build(:site_customization_page, slug: "as/as*la")
-    expect(custom_page).to be_invalid
+    expect(custom_page).not_to be_valid
   end
 
   it "dynamically validates the valid statuses" do