(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/server/lib/IBilling.ts

@@ -1,7 +1,8 @@
 import * as express from 'express';
+import {GristServer} from 'app/server/lib/GristServer';
 
 export interface IBilling {
-  addEndpoints(app: express.Express): void;
+  addEndpoints(app: express.Express, server: GristServer): void;
   addEventHandlers(): void;
   addWebhooks(app: express.Express): void;
   addMiddleware?(app: express.Express): Promise<void>;