(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/Limit.ts

@@ -0,0 +1,46 @@
+import {BaseEntity, Column, Entity, JoinColumn, ManyToOne, PrimaryGeneratedColumn} from 'typeorm';
+import {BillingAccount} from 'app/gen-server/entity/BillingAccount';
+import {nativeValues} from 'app/gen-server/lib/values';
+
+@Entity('limits')
+export class Limit extends BaseEntity {
+  @PrimaryGeneratedColumn()
+  public id: number;
+
+  @Column()
+  public limit: number;
+
+  @Column()
+  public usage: number;
+
+  @Column()
+  public type: string;
+
+  @Column({name: 'billing_account_id'})
+  public billingAccountId: number;
+
+  @ManyToOne(type => BillingAccount)
+  @JoinColumn({name: 'billing_account_id'})
+  public billingAccount: BillingAccount;
+
+  @Column({name: 'created_at', default: () => "CURRENT_TIMESTAMP"})
+  public createdAt: Date;
+
+  /**
+   * Last time the Limit.limit value was changed, by an upgrade or downgrade. Null if it has never been changed.
+   */
+  @Column({name: 'changed_at', type: nativeValues.dateTimeType, nullable: true})
+  public changedAt: Date|null;
+
+  /**
+   * Last time the Limit.usage was used (by sending a request to the model). Null if it has never been used.
+   */
+  @Column({name: 'used_at', type: nativeValues.dateTimeType, nullable: true})
+  public usedAt: Date|null;
+
+  /**
+   * Last time the Limit.usage was reset, probably by billing cycle change. Null if it has never been reset.
+   */
+  @Column({name: 'reset_at', type: nativeValues.dateTimeType, nullable: true})
+  public resetAt: Date|null;
+}