(core) Billing for formula assistant

Summary:
Adding limits for AI calls and connecting those limits with a Stripe Account.

- New table in homedb called `limits`
- All calls to the AI are not routed through DocApi and measured.
- All products now contain a special key `assistantLimit`, with a default value 0
- Limit is reset every time the subscription has changed its period
- The billing page is updated with two new options that describe the AI plan
- There is a new popup that allows the user to upgrade to a higher plan
- Tiers are read directly from the Stripe product with a volume pricing model

Test Plan: Updated and added

Reviewers: georgegevoian, paulfitz

Reviewed By: georgegevoian

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3907

app/client/ui/NotifyUI.ts

@@ -30,6 +30,10 @@ function buildAction(action: NotifyAction, item: Notification, options: IBeaconO
         return dom('a', cssToastAction.cls(''), t("Upgrade Plan"), {target: '_blank'},
           {href: commonUrls.plans});
       }
+    case 'manage':
+      if (urlState().state.get().billing === 'billing') { return null; }
+      return dom('a', cssToastAction.cls(''), t("Manage billing"), {target: '_blank'},
+        {href: urlState().makeUrl({billing: 'billing'})});
     case 'renew':
       // If already on the billing page, nothing to return.
       if (urlState().state.get().billing === 'billing') { return null; }