(core) Product update popups and hosted stripe integration

Summary:
- Showing nudge to individual users to sign up for free team plan.
- Implementing billing page to upgrade from free team to pro.
- New modal with upgrade options and free team site signup.
- Integrating Stripe-hosted UI for checkout and plan management.

Test Plan: updated tests

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D3456

stubs/app/client/ui/ProductUpgrades.ts

@@ -0,0 +1,27 @@
+import type {AppModel} from 'app/client/models/AppModel';
+import {commonUrls} from 'app/common/gristUrls';
+import {Disposable} from 'grainjs';
+
+export function buildUpgradeNudge(options: {
+  onClose: () => void;
+  onUpgrade: () => void
+}) {
+  return null;
+}
+
+export function buildNewSiteModal(owner: Disposable, current: string | null) {
+  window.location.href = commonUrls.plans;
+}
+
+export function buildUpgradeModal(owner: Disposable, planName: string)  {
+  window.location.href = commonUrls.plans;
+}
+
+export class UpgradeButton extends Disposable {
+  constructor(appModel: AppModel) {
+    super();
+  }
+  public buildDom() {
+    return null;
+  }
+}