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/system/polls/questions_spec.rb

@@ -20,7 +20,7 @@ describe "Poll Questions" do
 
     within "#poll_more_info_answers" do
       expect(page).to have_content "Pedestrian road"
-      expect(page).to have_selector "img[alt='Trees on both sides of the road']"
+      expect(page).to have_css "img[alt='Trees on both sides of the road']"
     end
   end
 end