New deploy procedure

- Deploying to staging is a normal push
- Deploying to production releases the current slug from staging

I just used this to (migrate-)deploy latest to production.
IIUC, there has never been a difference between this and an API-based deployment. Once we got a PX dyno and a 30m timeout extension, both would always work. Without a PX dyno / 30m extension, both would fail.

We were using the API because it let us build on staging then release to production. By moving to the copy-slug approach for production, there's no reason to keep the API stuff around for staging, just do a normal push.