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/budgets_spec.rb

@@ -101,7 +101,7 @@ describe "Admin budgets", :admin do
       click_link "Create multiple headings budget"
 
       expect(page).to have_select("Final voting style", selected: "Knapsack")
-      expect(page).not_to have_selector("#budget_hide_money")
+      expect(page).not_to have_css "#budget_hide_money"
 
       fill_in "Name", with: "Budget hide money"
       select "Approval", from: "Final voting style"