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/components/budgets/supports_info_component_spec.rb

@@ -10,7 +10,7 @@ describe Budgets::SupportsInfoComponent do
 
     render_inline component
 
-    expect(page).to have_selector ".supports-info"
+    expect(page).to have_css ".supports-info"
     expect(page).to have_content "It's time to support projects!"
     expect(page).to have_link "Keep scrolling to see all ideas"
   end