Use sandbox as parent for personal deployments

[amarjandu]

There might be a way to utilize the parent env functionality in scripts/export_environment.py to allow for a personal deployment to derive the majority of its values from deployments/sandbox/environment.py file.

This could be done by:

1. setting the directory name for a personal deployment to sandbox.foo
2. add personal changes to the deployment/foo/environment.py file
3. add personal local changes to the deployment/foo/environment.local.py file

We should spike to see if this is possible.

[hannes-ucsc]

This would use the existing mechanism added for dev.gitlab and prod.gitlab.