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/widgets/cards_spec.rb

@@ -199,11 +199,11 @@ describe "Cards", :admin do
         visit custom_page.url
 
         within("#widget_card_#{card_1.id}") do
-          expect(page).to have_selector("span", text: "MY LABEL")
+          expect(page).to have_css "span", text: "MY LABEL"
         end
 
         within("#widget_card_#{card_2.id}") do
-          expect(page).not_to have_selector("span")
+          expect(page).not_to have_css "span"
         end
       end