(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/common/Features.ts

@@ -58,6 +58,11 @@ export interface Features {
                                                 // for attached files in a document
 
   gracePeriodDays?: number;  // Duration of the grace period in days, before entering delete-only mode
+
+  baseMaxAssistantCalls?: number; // Maximum number of AI assistant calls. Defaults to 0 if not set, use -1 to indicate
+                                  // unbound limit. This is total limit, not per month or per day, it is used as a seed
+                                  // value for the limits table. To create a per-month limit, there must be a separate
+                                  // task that resets the usage in the limits table.
 }
 
 // Check whether it is possible to add members at the org level.  There's no flag