(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

app/common/Features.ts

@@ -9,6 +9,7 @@ export interface Product {
   features: Features;
 }
 
+
 // A product is essentially a list of flags and limits that we may enforce/support.
 export interface Features {
   vanityDomain?: boolean;   // are user-selected domains allowed (unenforced) (default: true)
@@ -69,5 +70,5 @@ export function canAddOrgMembers(features: Features): boolean {
 
 // Returns true if `product` is free.
 export function isFreeProduct(product: Product): boolean {
-  return ['starter', 'teamFree'].includes(product.name);
+  return ['starter', 'teamFree', 'Free'].includes(product?.name);
 }