(core) Removing temporary pro site

Summary: Creating a pro team site after Stripe checkout. Previously a stub site was always created and never removed, even if a user cancels the checkout process, which resulted in multiple 'ghost' sites that can't be removed.

Test Plan: Updated and added

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3985

app/common/BillingAPI.ts

@@ -229,7 +229,8 @@ export class BillingAPIImpl extends BaseAPI implements BillingAPI {
       body: JSON.stringify({
         domain,
         name,
-        planType: 'team'
+        planType: 'team',
+        next: window.location.href
       })
     });
     return data.checkoutUrl;