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
