(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

stubs/app/client/ui/ActivationPage.ts

@@ -0,0 +1,12 @@
+import {AppModel} from 'app/client/models/AppModel';
+import {Disposable} from 'grainjs';
+
+export class ActivationPage extends Disposable {
+  constructor(_appModel: AppModel) {
+    super();
+  }
+
+  public buildDom() {
+    return null;
+  }
+}

stubs/app/client/ui/BillingButtons.ts

@@ -0,0 +1,16 @@
+import {AppModel} from 'app/client/models/AppModel';
+import {DomElementArg} from 'grainjs';
+
+export function buildUserMenuBillingItem(
+  _appModel: AppModel,
+  ..._args: DomElementArg[]
+) {
+  return null;
+}
+
+export function buildAppMenuBillingItem(
+  _appModel: AppModel,
+  ..._args: DomElementArg[]
+) {
+  return null;
+}

stubs/app/client/ui/BillingPage.ts

@@ -0,0 +1,12 @@
+import {AppModel} from 'app/client/models/AppModel';
+import {Disposable} from 'grainjs';
+
+export class BillingPage extends Disposable {
+  constructor(_appModel: AppModel) {
+    super();
+  }
+
+  public buildDom() {
+    return null;
+  }
+}