Use system specs instead of feature specs

We get rid of database cleaner, and JavaScript tests are faster because
between tests we now rollback transactions instead of truncating the
database.

spec/lib/tasks/sitemap_spec.rb

@@ -1,6 +1,6 @@
 require "rails_helper"
 
-describe "rake sitemap:create", type: :feature do
+describe "rake sitemap:create", type: :system do
   let(:file) { Rails.root.join("public", "sitemap.xml") }
 
   before do