(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/SiteSwitcher.ts

@@ -1,9 +1,9 @@
-import {commonUrls, getSingleOrg, shouldHideUiElement} from 'app/common/gristUrls';
+import {getSingleOrg, shouldHideUiElement} from 'app/common/gristUrls';
 import {getOrgName} from 'app/common/UserAPI';
 import {dom, makeTestId, styled} from 'grainjs';
 import {AppModel} from 'app/client/models/AppModel';
 import {urlState} from 'app/client/models/gristUrlState';
-import {menuDivider, menuIcon, menuItemLink, menuSubHeader} from 'app/client/ui2018/menus';
+import {menuDivider, menuIcon, menuItem, menuItemLink, menuSubHeader} from 'app/client/ui2018/menus';
 import {icon} from 'app/client/ui2018/icons';
 import {colors} from 'app/client/ui2018/cssVars';
 
@@ -39,8 +39,8 @@ export function buildSiteSwitcher(appModel: AppModel) {
         testId('org'),
       )
     ),
-    menuItemLink(
-      { href: commonUrls.createTeamSite },
+    menuItem(
+      () => appModel.showNewSiteModal(),
       menuIcon('Plus'),
       'Create new team site',
       testId('create-new-site'),