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/views/welcome/index.html.erb_spec.rb

@@ -15,7 +15,7 @@ RSpec.describe "welcome#index" do
                      btn_path_link: debates_path(order: "recommendations") }
 
     within 'li[data-slide="0"] .card' do
-      expect(page).to have_selector("img")
+      expect(page).to have_css "img"
     end
   end
 
@@ -33,7 +33,7 @@ RSpec.describe "welcome#index" do
                      btn_path_link: debates_path(order: "recommendations") }
 
     within 'li[data-slide="0"] .card' do
-      expect(page).not_to have_selector("img")
+      expect(page).not_to have_css "img"
     end
   end
 end