(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/ui2018/cssVars.ts

@@ -142,6 +142,7 @@ export const vars = {
   onboardingPopupZIndex: new CustomProp('onboarding-popup-z-index', '1000'),
   floatingPopupZIndex: new CustomProp('floating-popup-z-index', '1002'),
   tutorialModalZIndex: new CustomProp('tutorial-modal-z-index', '1003'),
+  pricingModalZIndex: new CustomProp('pricing-modal-z-index', '1004'),
   notificationZIndex: new CustomProp('notification-z-index', '1100'),
   browserCheckZIndex: new CustomProp('browser-check-z-index', '5000'),
   tooltipZIndex: new CustomProp('tooltip-z-index', '5000'),