Update Heroku vars for xPRO
Update Heroku vars for xPRO

* Update MITXPRO_BASE_URL and MITXPRO_SECURE_SSL_HOST for mitxpro-rc to
  use Fastly CDN's domain
* Add CSRF_TRUSTED_ORIGINS for RC and production. It had already been
  set in production, but had not been added here.

mitodl/salt-ops,mitodl/salt-ops