Add support of _FILE environment variables for secrets

[soyouzpanda]

Feature Request

Add support of _FILE environment variables for secrets.

As discussed here: NixOS/nixpkgs#401798 (comment), we need to pass the secrets, like DJANGO_SECRET_KEY, COLLABORATION_SERVER_SECRET, etc. in files.
The current solution is to wrap the gunicorn/celery/collaboration server in a script that export environment variables after cat-ing the files.

But we think that the support of secrets in files should be instead supported by docs, through environment variables like DJANGO_SECRET_KEY_FILE, COLLABORATION_SERVER_SECRET_FILE, etc.

[mweinelt]

Passing them around as files is more compatible with the systemd credential API and can rely on the filesystem discretionary access control, which is what we would prefer to use in NixOS.