(core) Add basic activation page to grist-ee

Summary:
Adds an activation page to grist-ee that currently shows activation status.

Follow-up diffs will introduce additional functionality, such as the ability to
enter activation keys directly from the activation page.

Test Plan: No grist-ee tests (yet).

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: dsagal, paulfitz

Differential Revision: https://phab.getgrist.com/D3582

app/client/billingMain.ts

@@ -0,0 +1,5 @@
+import {BillingPage} from 'app/client/ui/BillingPage';
+import {setupPage} from 'app/client/ui/setupPage';
+import {dom} from 'grainjs';
+
+setupPage((appModel) => dom.create(BillingPage, appModel));