Provide DNS for staging site

[mgwalker]

Currently the staging site is located at http://open-forest-staging.app.cloud.gov. It should be hosted under a USDA or USFS domain instead, similar to production. There are two primary reasons:

1. The GSA TTS Digital Council has asked us to reduce the number of public-facing staging sites hosted on our domains, including staging sites on cloud.gov. We can continue to use the cloud.gov domain by adding a justification for keeping it, but it's possible that's just a short-term bandaid until someone decides later to change the rules.

2. The staging and production sites having different DNS configurations can result in bugs that appear in one environment but not the other. For example, the cookie/login bug showed up in production but not staging, because in staging, the API DNS was not considered 3rd-party from the frontend DNS and the cookies were fine. In production, however, they are hosted at different domains so the cookies wouldn't persist.

@bboddiger and @mwreiss - can we discuss at our next engineering sync?