(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

stubs/app/client/ui/ActivationPage.ts

@@ -5,3 +5,8 @@ import {
 export function getActivationPage(): IActivationPageCreator {
   return DefaultActivationPage;
 }
+
+export function showEnterpriseToggle() {
+  // To be changed by enterprise module
+  return false;
+}