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/site_customization/documents_spec.rb

@@ -39,7 +39,7 @@ describe "Documents", :admin do
 
     visit admin_site_customization_documents_path
 
-    expect(page).to have_selector("#documents .document-row", count: per_page)
+    expect(page).to have_css "#documents .document-row", count: per_page
 
     within("ul.pagination") do
       expect(page).to have_content("1")
@@ -48,7 +48,7 @@ describe "Documents", :admin do
       click_link "Next", exact: false
     end
 
-    expect(page).to have_selector("#documents .document-row", count: 2)
+    expect(page).to have_css "#documents .document-row", count: 2
   end
 
   scenario "Create" do