(core) Customizable stripe plans.

Summary:
- Reading plans from Stripe, and allowing Stripe to define custom plans.
- Storing product features (aka limits) in Stripe, that override those in db.
- Adding hierarchical data in Stripe. All features are defined at Product level but can be overwritten on Price levels.
- New options for Support user to
-- Override product for team site (if he is added as a billing manager)
-- Override subscription and customer id for a team site
-- Attach an "offer", an custom plan configured in stripe that a team site can use
-- Enabling wire transfer for subscription by allowing subscription to be created without a payment method (which is customizable)

Test Plan: Updated and new.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D4201

app/gen-server/migration/1711557445716-Billing.ts

@@ -0,0 +1,23 @@
+import {nativeValues} from 'app/gen-server/lib/values';
+import {MigrationInterface, QueryRunner, TableColumn} from 'typeorm';
+
+export class Billing1711557445716 implements MigrationInterface {
+  public async up(queryRunner: QueryRunner): Promise<any> {
+    await queryRunner.addColumn('billing_accounts', new TableColumn({
+      name: 'features',
+      type: nativeValues.jsonType,
+      isNullable: true,
+    }));
+
+    await queryRunner.addColumn('billing_accounts', new TableColumn({
+      name: 'payment_link',
+      type: nativeValues.jsonType,
+      isNullable: true,
+    }));
+  }
+
+  public async down(queryRunner: QueryRunner): Promise<any> {
+    await queryRunner.dropColumn('billing_accounts', 'features');
+    await queryRunner.dropColumn('billing_accounts', 'payment_link');
+  }
+}