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/budget_headings_spec.rb

@@ -37,7 +37,7 @@ describe "Admin budget headings", :admin do
 
       within "section", text: "Heading groups" do
         within "tbody" do
-          expect(page).to have_selector "tr", count: 3
+          expect(page).to have_css "tr", count: 3
 
           within("tr", text: "Laptops") { expect(page).to have_content "€1,000" }
           within("tr", text: "Tablets") { expect(page).to have_content "€2,000" }