(core) Restoring GRIST_DEFAULT_PRODUCT functionality

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

app/client/ui/CreateTeamModal.ts

@@ -136,7 +136,9 @@ function buildTeamPage({
       }
       await create();
     } finally {
-      disabled.set(false);
+      if (!disabled.isDisposed()) {
+        disabled.set(false);
+      }
     }
   }
   const clickOnEnter = dom.onKeyPress({