(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/gen-server/ApiServer.ts

@@ -73,7 +73,7 @@ export function addOrg(
   userId: number,
   props: Partial<OrganizationProperties>,
   options?: {
-    planType?: 'free'
+    planType?: string,
   }
 ): Promise<number> {
   return dbManager.connection.transaction(async manager => {