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/questions_spec.rb

@@ -51,8 +51,8 @@ describe "Admin poll questions", :admin do
       click_link "Create question"
 
       expect(page).to have_content("Create question to poll Movies")
-      expect(page).to have_selector("input[id='poll_question_poll_id'][value='#{poll.id}']",
-                                    visible: :hidden)
+      expect(page).to have_css "input[id='poll_question_poll_id'][value='#{poll.id}']",
+                               visible: :hidden
 
       fill_in "Question", with: "Star Wars: Episode IV - A New Hope"
       click_button "Save"