Create a separate testing instance for deploy previews

[JackWilb]

The current config only specifies the production system, but for PRs it would be nice to have a full testing infrastructure (heroku, aws, etc.)

The clients would just need to be modified to use the testing API using one env var

[waxlamp]

In case it's helpful, take a look at https://github.com/dandi/dandi-infrastructure to see how we've set this workflow up for a different project.

[JackWilb]

That is helpful, notably https://github.com/dandi/dandi-infrastructure/blob/master/terraform/staging_pipeline.tf