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/booth_assigments_spec.rb

@@ -222,10 +222,10 @@ describe "Admin booths assignments", :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).not_to have_selector "#recounts_list"
+      expect(page).not_to have_css "#recounts_list"
     end
 
     scenario "Results for a booth assignment" do