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/admin/poll/polls_spec.rb

@@ -327,11 +327,11 @@ describe "Admin polls", :admin do
             expect(page).to have_content("10")
           end
 
-          expect(page).not_to have_selector "#total_system"
+          expect(page).not_to have_css "#total_system"
         end
 
-        expect(page).to have_selector "#poll_booth_assignment_#{booth_assignment.id}_recounts"
-        expect(page).not_to have_selector "#poll_booth_assignment_#{booth_assignment.id}_system"
+        expect(page).to have_css "#poll_booth_assignment_#{booth_assignment.id}_recounts"
+        expect(page).not_to have_css "#poll_booth_assignment_#{booth_assignment.id}_system"
       end
     end
   end