(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/server/server.ts

@@ -91,7 +91,7 @@ export async function main() {
       }, {
         setUserAsOwner: false,
         useNewPlan: true,
-        planType: 'free'
+        planType: 'teamFree'
       });
     }
   }