Add and apply Capybara/RSpec/HaveSelector rule

This rule was added in rubocop-capybara 2.19.0. We were following it
about 85% of the time.

Now we won't have to check both have_css and have_selector when
searching the code.

spec/components/users/public_activity_component_spec.rb

@@ -34,7 +34,7 @@ describe Users::PublicActivityComponent, controller: UsersController do
         with_request_url user_path(user, filter: "follows") do
           render_inline component
 
-          expect(page).to have_selector "li.is-active", text: "1 Following"
+          expect(page).to have_css "li.is-active", text: "1 Following"
         end
       end
     end