(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/server/lib/sendAppPage.ts

@@ -96,6 +96,7 @@ export function makeGristConfig(options: MakeGristConfigOptions): GristLoadConfi
     userLocale: (req as RequestWithLogin | undefined)?.user?.options?.locale,
     telemetry: server?.getTelemetry().getTelemetryConfig(req as RequestWithLogin | undefined),
     deploymentType: server?.getDeploymentType(),
+    forceEnableEnterprise: isAffirmative(process.env.GRIST_FORCE_ENABLE_ENTERPRISE),
     templateOrg: getTemplateOrg(),
     onboardingTutorialDocId: getOnboardingTutorialDocId(),
     canCloseAccount: isAffirmative(process.env.GRIST_ACCOUNT_CLOSE),