(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/client/ui/AccountWidget.ts

@@ -108,7 +108,7 @@ export class AccountWidget extends Disposable {
           menuItemLink(urlState().setLinkUrl({billing: 'billing'}), 'Billing Account') :
           menuItem(() => null, 'Billing Account', dom.cls('disabled', true))
         ) :
-        menuItemLink({href: commonUrls.plans}, 'Upgrade Plan'),
+        menuItem(() => this._appModel.showUpgradeModal(), 'Upgrade Plan'),
 
       mobileModeToggle,