(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/gen-server/entity/BillingAccount.ts

@@ -3,12 +3,13 @@ import {BillingAccountManager} from 'app/gen-server/entity/BillingAccountManager
 import {Organization} from 'app/gen-server/entity/Organization';
 import {Product} from 'app/gen-server/entity/Product';
 import {nativeValues} from 'app/gen-server/lib/values';
+import {Limit} from 'app/gen-server/entity/Limit';
 
 // This type is for billing account status information.  Intended for stuff
 // like "free trial running out in N days".
-interface BillingAccountStatus {
+export interface BillingAccountStatus {
   stripeStatus?: string;
-  currentPeriodEnd?: Date;
+  currentPeriodEnd?: string;
   message?: string;
 }
 
@@ -68,6 +69,9 @@ export class BillingAccount extends BaseEntity {
   @OneToMany(type => Organization, org => org.billingAccount)
   public orgs: Organization[];
 
+  @OneToMany(type => Limit, limit => limit.billingAccount)
+  public limits: Limit[];
+
   // A calculated column that is true if it looks like there is a paid plan.
   @Column({name: 'paid', type: 'boolean', insert: false, select: false})
   public paid?: boolean;