Summary: The GRIST_DEFAULT_PRODUCT wasn't used for grist-ee, now it is respected.
Test Plan:
I've build grist-ee docker image from github and run it using our instruction (both for recreating the issue and confirming it is fixed)
```
docker run -p 8484:8484 \
-v $PWD:/persist \
-e GRIST_SESSION_SECRET=invent-a-secret-here \
-e GRIST_SINGLE_ORG=cool-beans
-it gristlabs/grist-ee
```
For grist-core I recreated/confirmed it is fixed it just by `GRIST_SINGLE_ORG=team npm start` in the core folder.
I also created some team sites using stubbed UI and confirmed that they were using the GRIST_DEFAULT_PRODUCT product.
Reviewers: paulfitz
Reviewed By: paulfitz
Subscribers: paulfitz
Differential Revision: https://phab.getgrist.com/D4271