Turn default services into a configurable setting

With the default value being all enabled services

spec/models/user_spec.rb

@@ -178,7 +178,7 @@ RSpec.describe User, type: :model do
     after { clear_enqueued_jobs }
 
     it "enables default services" do
-      expect(user).to receive(:enable_service).with(%w[ discourse gitea mastodon mediawiki remotestorage xmpp ])
+      expect(user).to receive(:enable_service).with(Setting.default_services)
       user.send :devise_after_confirmation
     end