(core) AdminPanel: hide the enterprise toggle in core and grist-ee images

Summary:
In the pure OSS `grist-oss` image, the `ActivationPage` module from
stubs is used, as the `ext` code is completely missing. We can easily
just always return `false` here.

In the case when the `ext` directory exists, this may mean we're in
the standard `grist` image or the `grist-ee` image. The latter is
distinguished by having `GRIST_FORCE_ENABLE_ENTERPRISE` so we check if
that's on, and hide the toggle accordingly if so.

Test Plan:
Use these changes to build the three Docker images
(`grist-oss`, `grist`, and `grist-ee`) and verify that only `grist`
shows the toggle.

Reviewers: jarek

Reviewed By: jarek

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D4321

app/common/gristUrls.ts

@@ -810,6 +810,9 @@ export interface GristLoadConfig {
   // The Grist deployment type (e.g. core, enterprise).
   deploymentType?: GristDeploymentType;
 
+  // Force enterprise deployment? For backwards compatibility with grist-ee Docker image
+  forceEnableEnterprise?: boolean;
+
   // The org containing public templates and tutorials.
   templateOrg?: string|null;