config: re-enable WTF_CSRF_ENABLED for staging
config: re-enable WTF_CSRF_ENABLED for staging

this was presumably disabled to allow gatling scripts to run, but we now
have a mechanism in place that allows gatling to work properly with csrf

alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend