(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({

app/gen-server/entity/Product.ts

@@ -147,12 +147,11 @@ export const PRODUCTS: IProduct[] = [
  */
 export function getDefaultProductNames() {
   const defaultProduct = process.env.GRIST_DEFAULT_PRODUCT;
-  const personalFreePlan = PERSONAL_FREE_PLAN;
   return {
     // Personal site start off on a functional plan.
-    personal: defaultProduct || personalFreePlan,
+    personal: defaultProduct || PERSONAL_FREE_PLAN,
      // Team site starts off on a limited plan, requiring subscription.
-    teamInitial: defaultProduct || 'stub',
+    teamInitial: defaultProduct || STUB_PLAN,
     // Team site that has been 'turned off'.
     teamCancel: 'suspended',
     // Functional team site.